A conditional sentence describes a condition and its result: if one thing happens, another thing follows. English has a small set of fixed patterns for doing this, and once you can name the four types, most of the confusion disappears. The trouble is rarely the meaning. It is keeping the tenses in the right clause.

The structure of a conditional

Every conditional has two parts:

• the if-clause (the condition): if it rains
• the main clause (the result): I will stay home

The order can swap with no change in meaning. There is one punctuation rule, and it is worth getting right because it is easy and examiners notice it:

• If the if-clause comes first, use a comma. If it rains, I will stay home.
• If the if-clause comes second, use no comma. I will stay home if it rains.

That is the whole punctuation rule. Now the four patterns.

The four conditionals at a glance

Read the table top to bottom and you can see the engine: as the situation gets less real, the tense in each clause steps one notch further back in time.

Zero conditional: facts and general truths

Use the zero conditional for things that are always true. The result is not a prediction; it is a law of how the world works. Both clauses are in the present simple.

if + present simple, present simple

You can usually replace if with when here without changing the meaning, because the result happens every time: when you heat ice, it melts.

First conditional: real future situations

Use the first conditional for a future situation you think is realistic or likely. The condition is in the present simple, and the result uses will.

if + present simple, will + base verb

Note the present tense in the if-clause. The condition is in the future in your head, but English uses the present simple to express it. You can soften the result with might or could when you are less sure: if it rains, I might stay home.

Second conditional: unreal or unlikely present and future

Use the second conditional for situations that are imaginary, unlikely, or contrary to fact right now. The condition is in the past simple, but the meaning is present or future, not past.

if + past simple, would + base verb

With the verb be, the standard second-conditional form is were for every subject, not was: If I were you, I would wait. If she were here, she would know. You will hear if I was constantly in speech and it is widely accepted, but if I were is the form to use in writing and exams. As with the first conditional, could or might can replace would: if I won the lottery, I could buy a house.

Third conditional: the unreal past

Use the third conditional to talk about a past situation that did not happen, and to express regret about it. Both clauses look further back: past perfect in the condition, would have plus a past participle in the result.

if + past perfect, would have + past participle

Everything here is hypothetical, because the past is fixed. If I had studied, I would have passed tells you the speaker did not study and did not pass. Again, could have and might have are available: if I had studied, I might have passed.

Mixed conditionals

Sometimes the condition and the result sit in different times. The most common mix is a past condition with a present result: something that did or did not happen in the past still affects now.

if + past perfect, would + base verb

• If I had taken the job, I would be richer now. (Past decision, present consequence.)
• If she had studied medicine, she would be a doctor today.

The reverse also exists - a present condition with a past result - but it is rarer: if he weren't so careless, he wouldn't have lost the keys. The point is that you are free to mix the clauses when the timeline genuinely crosses; you do not have to keep both halves in the same conditional.

Unless, and other ways in

Unless means if not. It introduces the condition under which the result will not happen:

• I'll stay home unless it stops raining. (= if it does not stop raining)
• Unless you hurry, we'll miss the train. (= if you do not hurry)

It takes the same comma rule and the same tense pattern as a normal if-clause. You can also begin conditions with as long as, provided that, and in case, but if and unless carry the vast majority of the work.

Common mistakes

Ranked by how often learners actually make them.

1. Putting will or would in the if-clause. This is the big one. The condition never takes will or would. If I will see him, I'll tell him should be If I see him, I'll tell him. And If I would have known, I would have come should be If I had known, I would have come. Keep will and would in the result clause only.
2. Mixing up the second and third conditional. If I would have studied, I would have passed blends the two. The third conditional needs the past perfect in the condition: If I had studied, I would have passed. Use the second for the unreal present, the third for the unreal past.
3. Comma errors. A comma is needed only when the if-clause comes first. I will stay home, if it rains is wrong; drop the comma when the condition comes second. And do not forget the comma when it does come first.
4. Using was instead of were in the second conditional. If I was rich is informal at best. In careful English it is If I were rich.
5. Confusing zero and first. If you heat ice, it will melt turns a general truth into a one-off prediction. For a fact, keep both clauses present: if you heat ice, it melts.

Practice

Choose or complete the correct form. Answers are below.

1. If you ___ (mix) blue and yellow, you get green.
2. If it ___ (rain) tomorrow, we will cancel the picnic.
3. If I ___ (be) you, I would take the job.
4. If she had left earlier, she ___ (catch) the train.
5. I won't forgive him ___ he apologises.

Answers: 1. mix (zero conditional - a general truth) 2. rains (first conditional - present simple in the if-clause, never will) 3. were (second conditional - standard were for all subjects) 4. would have caught (third conditional - would have + past participle) 5. unless (unless = if not)