What Are Conditionals?
A conditional sentence describes a situation and its result — what happens if something else is true or occurs. Every conditional sentence has two parts:
- The if-clause (also called the condition): the situation or requirement
- The main clause (also called the result): what happens because of that situation
English has several types of conditional sentences. This guide focuses on the two most common for everyday communication: the zero conditional and the first conditional. Both use the word if, but they express very different ideas — one describes permanent facts, and the other describes real possibilities in the future.
Learning these two structures will immediately improve your speaking and writing, because you will use them constantly: giving advice, making plans, describing rules, and talking about future possibilities.
Think of conditionals as "cause and effect" sentences. The if-clause is the cause; the main clause is the effect. Once you see them this way, the grammar becomes much easier to remember.
Zero Conditional: Facts and General Truths
The zero conditional is used to talk about things that are always true — scientific facts, natural laws, habits, and general rules. The result is 100% certain whenever the condition is met. It is called "zero" because there is zero uncertainty about the outcome.
Structure: If + present simple, present simple
If you heat water to 100°C, it boils. If it rains, the ground gets wet. If you mix red and blue, you get purple. If babies are hungry, they cry. If you don't water plants, they die.
Notice that both the if-clause and the main clause use the present simple tense. The verb in each clause describes a routine, a fact, or a natural consequence — not a specific event in the future.
You can also use the zero conditional to describe personal habits and routines:
If I miss breakfast, I feel tired by mid-morning. If the office is too hot, people lose concentration. If you press this button, the machine starts.
A useful test: if you can replace if with when or whenever without changing the meaning, you are probably looking at a zero conditional. "When you heat water to 100°C, it boils" means exactly the same thing.
Use the zero conditional when the result is a universal truth or a certainty — not just a possibility. If you could imagine the outcome not happening sometimes, you probably need a different conditional.
First Conditional: Real Future Situations
The first conditional describes a real, possible situation in the future and its likely result. The speaker believes the condition genuinely could happen — it is not imaginary or impossible.
Structure: If + present simple, will + base verb
If it rains tomorrow, I will stay at home. If you study hard, you will pass the exam. If she calls me, I will tell her the news. If we leave now, we will catch the train. If the weather is nice this weekend, we will have a picnic.
The key point is that you use the present simple in the if-clause — not a future form. This surprises many learners, but English treats the if-clause as a kind of condition that is already framed in the present, even though the situation refers to the future.
In the main clause, will is the standard choice, but other modal verbs are also possible depending on the meaning you want:
- can — for ability or permission: If you finish early, you can leave.
- might / may — for less certain results: If it stays cloudy, it might snow.
- should — for advice or recommendation: If you feel unwell, you should see a doctor.
- must — for obligation: If you enter the building, you must sign in.
If you ask politely, he might help you. If the test is difficult, you should take your time. If visitors arrive, they must wear a badge. If she applies for the job, she could get an interview.
A quick memory trick: "If + present, will + base." Never put will in the if-clause. The if-clause always stays in the present simple, even when you are talking about the future.
If vs When and Unless
Once you are comfortable with basic conditional sentences, three small words can add a lot of precision: when, unless, and their relationship to if.
If vs When
Both if and when can introduce conditional-style clauses, but they carry different meanings:
- If suggests that something is possible but not certain. The speaker is not sure whether it will happen.
- When suggests that something is certain or expected. The speaker believes it will definitely happen.
If I get a promotion, I will celebrate. (I'm not sure I will get one.) When I get a promotion, I will celebrate. (I'm confident I will get one.) If the bus is late, we will take a taxi. (The bus might or might not be late.) When the bus arrives, get on quickly. (We know the bus is coming.)
In zero conditional sentences, you can usually use if or when interchangeably because the result is always true. In first conditional sentences, choose the word that best reflects your certainty about the situation.
Unless
Unless means if not. It introduces a negative condition and is a very natural word in both zero and first conditional sentences. You do not add a separate not after unless.
Unless you hurry, we will miss the flight. (= If you don't hurry, we will miss the flight.) Unless it rains, the match will go ahead. (= If it doesn't rain, the match will go ahead.) Unless you heat it, ice doesn't melt. (= If you don't heat it, ice doesn't melt.)
Never write "unless ... not" together. Unless already contains the negative meaning. "Unless you don't hurry" is a double negative and confuses the meaning entirely.
Punctuation and Word Order
Good punctuation makes your conditional sentences easier to read, and word order gives you flexibility in how you express your ideas. Both are worth understanding clearly.
Comma or No Comma?
The rule is straightforward:
- When the if-clause comes first, use a comma before the main clause.
- When the main clause comes first, no comma is needed.
If it is cold, I will wear a coat. (comma — if-clause first) I will wear a coat if it is cold. (no comma — main clause first) If you practise every day, your English will improve. Your English will improve if you practise every day. Unless you book early, the hotel will be full. The hotel will be full unless you book early.
Both word orders are grammatically correct and mean exactly the same thing. In spoken English, the order you choose often depends on what information you want to emphasise. Starting with the if-clause highlights the condition; starting with the main clause highlights the result.
Word Order Inside Each Clause
Each clause follows normal English word order: Subject + Verb + Object/Complement. Do not move the subject or verb around just because you are in a conditional sentence.
If she arrives early, she will help us set up. If the price drops, many people will buy it. We will cancel the event if it snows heavily.
A quick check: read your sentence aloud and pause where the comma is. If it sounds natural to pause there and the two parts each make sense on their own (as cause and effect), your structure is correct.
Common Mistakes to Avoid
Even advanced learners make mistakes with conditionals. Here are the most frequent errors — and how to fix them.
Mistake 1: Using will in the If-Clause
This is the single most common error. The if-clause always uses the present simple in zero and first conditionals — not will.
WRONG: If it will rain tomorrow, I will stay inside. RIGHT: If it rains tomorrow, I will stay inside. WRONG: If you will study hard, you will pass. RIGHT: If you study hard, you will pass.
Mistake 2: Using the Wrong Conditional Type
Zero conditional is for facts; first conditional is for future possibilities. Mixing them creates a grammatically incorrect or confusing sentence.
WRONG (zero used for future plan): If I finish work early, I go to the gym. RIGHT (first conditional for plan): If I finish work early, I will go to the gym. WRONG (first used for a fact): If you freeze water, it will turn to ice. RIGHT (zero for a fact): If you freeze water, it turns to ice.
Mistake 3: Writing "unless ... not"
As explained earlier, unless already means if not. Adding not creates a confusing double negative.
WRONG: Unless you don't study, you will fail. RIGHT: Unless you study, you will fail. (OR) If you don't study, you will fail.
Mistake 4: Forgetting the Comma
When the if-clause begins the sentence, always follow it with a comma before the main clause.
WRONG: If you are hungry I can make you something to eat. RIGHT: If you are hungry, I can make you something to eat.
Mistake 5: Using a Continuous Tense in the If-Clause
The if-clause in zero and first conditionals uses the simple form, not a continuous form, unless you specifically want to emphasise an ongoing action.
WRONG: If it is raining tomorrow, we will cancel the trip. RIGHT: If it rains tomorrow, we will cancel the trip. (Continuous is possible only when the ongoing nature matters: If she is still sleeping when he arrives, don't wake her.)
Before you submit any writing, highlight every conditional sentence and ask two questions: (1) Is there will in the if-clause? Remove it. (2) Does the comma follow the right pattern? Fix it if not. These two checks will catch the vast majority of errors.
Practice and Next Steps
The best way to master zero and first conditionals is to make them part of your everyday English. Here are some practical strategies that work well for learners at B1-C1 level.
Write your own examples. Think about your own life and write five zero conditional sentences about things that are always true for you — your habits, your environment, or facts you know well. Then write five first conditional sentences about your plans or hopes. Personalised examples are far easier to remember than textbook ones.
Zero: If I drink coffee in the evening, I cannot sleep. Zero: If the traffic is bad, my commute takes an hour. First: If I save enough money, I will travel next year. First: If my friend visits this weekend, we will go hiking.
Notice conditionals in the world around you. Instructions, warning signs, recipes, and advice columns are full of conditional sentences. Whenever you read something in English, pause when you see if, when, or unless, and identify which type of conditional it is.
Use them in conversation. When someone asks about your plans, try to use a first conditional: "If the weather is nice, I will go to the market." When you explain how something works, use a zero conditional: "If you press this, the screen lights up."
Move forward to the second and third conditionals. Once you feel confident with zero and first conditionals, the next step is to learn the second conditional (for imaginary present or future situations) and the third conditional (for past situations that did not happen). These build directly on what you have learned here.
Remember: making mistakes is a normal and necessary part of learning. Every time you catch yourself using will in an if-clause and correct it, you are strengthening your grammar. Keep practising, and these structures will become completely natural.
Frequently Asked Questions
What is the difference between zero and first conditional?
The zero conditional describes facts and general truths that are always true (If you heat ice, it melts). The first conditional describes real, possible future situations and their likely results (If it rains tomorrow, I will stay home). Zero conditional uses present simple in both clauses; first conditional uses present simple in the if-clause and will + base verb in the main clause.
Can I use 'will' in the if-clause of a first conditional?
No. This is the most common mistake learners make. The if-clause in a first conditional always uses the present simple tense, not will. Correct: 'If it rains, I will bring an umbrella.' Incorrect: 'If it will rain, I will bring an umbrella.'
When do I use 'if' and when do I use 'when' in conditional sentences?
Use 'if' when you are not sure whether the situation will happen (If she calls, I will answer). Use 'when' when you are confident or certain the situation will happen (When she calls, I will answer). In zero conditional sentences about facts, both if and when are interchangeable.
What does 'unless' mean in a conditional sentence?
'Unless' means 'if not'. It introduces a negative condition without using the word 'not'. For example, 'Unless you hurry, we will miss the bus' means 'If you do not hurry, we will miss the bus.' Never combine unless with not, as that creates a confusing double negative.
Do I need a comma in a conditional sentence?
Yes, but only in one position. When the if-clause comes at the beginning of the sentence, place a comma after it before the main clause: 'If it snows, the school will close.' When the main clause comes first, no comma is needed: 'The school will close if it snows.'
Can I use other modal verbs instead of 'will' in the first conditional?
Yes. While 'will' is the most common choice, you can use might or may for less certain outcomes, can for ability or permission, should for advice, and must for obligation. Examples: 'If you feel ill, you should rest.' 'If she applies, she might get the job.' 'If you enter, you must sign in.'