Zero & First Conditionals

Conditionals are "if" sentences, and almost everyone trips on the same spot: which tense goes in the if half. Your instinct says the sentence is about the future, so you reach for will on both sides and write "If it will rain, we will stay in." It feels right, and it's wrong. English has a quiet rule here that nobody explains clearly, so students guess. Once you see it, the guessing stops.

The zero and first conditionals both deal with things that are real or likely — facts, and things that might genuinely happen. That's what makes them the most useful pair to master first: they cover almost everything you say in ordinary life. No fantasy, no daydreams — just cause and effect, and realistic plans.

Zero conditional: things that are always true

Use it for facts and rules — here if means "whenever". Both halves are present simple:

If you heat ice, it melts. If the bell rings, we line up.

The zero conditional describes something that happens every time the condition is met. Water always melts when you heat it. You always line up when the bell goes. Because there's no doubt about the outcome, you can usually swap if for when and the meaning barely changes: When you heat ice, it melts.

First conditional: a likely future

Use it for something that could really happen once — a plan, a warning, a promise. The if half is present simple; the result half uses will:

If it rains, we 'll stay in. If you revise, you 'll pass.

Here the outcome isn't certain, it's likely. You don't know for sure it will rain — but if it does, staying in is the plan.

The pattern to remember

• Zero: If + present, ... present. (always true)
• First: If + present, ... will. (likely future)

Notice the if half is present in both — even when you're talking about the future. Never "If it will rain". Put the will in the result half only.

A second test: "always" or "just this once"?

Here's a quick way to choose between the two. Ask yourself whether the sentence is a general truth or a one-off. If you could add the word always or every time and it still makes sense, you want the zero conditional: If you drop a ball, it (always) bounces. If it's about one particular situation coming up, you want the first: If you drop that vase, it'll smash. One is a rule of the world; the other is a prediction about a single moment.

More worked examples

• ✓ If you mix red and white, you get pink. (a fact — always true → zero)
• ✗ If you will touch that, you will get burned. → ✓ If you touch that, you'll get burned. (keep the if-half present → first)
• ✓ If we miss the bus, we 'll walk. (one likely situation → first)
• ✗ If plants get no water, they will die. is fine as a warning, but if you mean the general rule, use zero: ✓ If plants get no water, they die.
• ✓ If you press this button, the light comes on. (how the thing works → zero)

Watch out — the comma

When the if half comes first, put a comma after it. When the result comes first, you don't need one:

If it rains, we'll stay in. → but → We'll stay in if it rains. (no comma)

Both orders mean exactly the same thing. You just move the comma to match where the if sits.

Edge case: unless, and other if-words

Unless is a handy one that means "if... not". "Unless you hurry, you'll be late" is the same as "If you don't hurry, you'll be late." It follows the first-conditional pattern: present tense after unless, will in the result. You'll also meet as soon as and when doing similar jobs — "As soon as the film ends, I'll text you." They all keep the same rule: no will in the condition half.

Why students muddle the two

The reason these get mixed up is that both talk about things that could happen, so it feels natural to treat them the same. The difference is really about certainty and repetition. The zero conditional describes a result that follows every single time, like a law of nature or a fixed rule, which is why both halves stay in the present simple and nothing about the future creeps in. The first conditional describes a result you expect this time if the condition is met, which is why the future will appears in the result. If you can feel whether you mean "this always happens" or "this will probably happen next", you'll pick the right one without even thinking about the labels. And here is the reassuring part: even when you slip, the meaning usually still gets through, so there is no need to freeze over the choice. Learn the shape once, and it becomes automatic surprisingly quickly.

Why it matters

These two patterns carry a huge amount of everyday English — instructions, warnings, promises, and the science answers your teachers love. In a science test especially, the zero conditional is exactly how you state a rule: "If you increase the temperature, the reaction speeds up." Get the tenses clean and your writing instantly sounds more controlled and sure of itself. Slip a will into the if half and a careful reader notices at once. The fix costs nothing once the pattern is muscle memory.

Quick check

Complete with the right form:

1. If you drop it, it ___ (break). (likely result)

2. If you mix blue and yellow, you ___ (get) green. (a fact)

3. If you ___ (heat) water to 100C, it boils. (always true)

4. If we win tomorrow, we ___ (be) top of the league. (likely future)

5. Fix it: If it will snow, school will close.

6. Rewrite with unless: If you don't leave now, you'll miss it.

Where next

• Second and Third Conditionals — for unreal and imagined situations.
• Talking About the Future — the will half.
• The Present Simple — the if half.