Conditionals are sentences with two clauses – an “if” clause and a main clause – that are closely related. Conditional sentences are often divided into different types.
1. The Zero Conditional (Present Real Conditional)
"Zero conditional" refers to conditional sentences that express a factual implication, rather than describing a hypothetical situation or potential future circumstance. The term is used particularly when both clauses are in the present tense; however such sentences can be formulated with a variety of tenses/moods, as appropriate to the situation.
- If you don't eat for a long time, you become hungry.
- If the alarm goes off, there's a fire somewhere in the building.
- If you are going to sit an exam tomorrow, go to bed early tonight!
- If aspirins will cure it, I'll take a couple tonight.
- If you make a mistake, someone lets you know.
2. The First Conditional (Present or Future Real Conditional)
"First conditional" refers to a pattern used in predictive conditional sentences, i.e. those that concern consequences of a probable future event. In the basic first conditional pattern, the condition is expressed using the present tense (having future meaning in this context). In some common fixed expressions or in old-fashioned or excessively formal, the present subjunctive is occasionally found. The consequence using the future construction with “will” (or “shall”).
- If need be, we'll rent a car.
- If you make a mistake, someone will let you know.
- If he asks me, I will/shall consider his proposal carefully.
3. The Second Conditional (Present Unreal Conditional)
"Second conditional" refers to a pattern used to describe hypothetical, typically counterfactual situations with a present or future time frame (for past time frames the third conditional is used). In the normal form of the second conditional, the condition clause is in the past tense (although it does not have past meaning. The consequence is expressed using the conditional construction with the auxiliary “would”.
- If I liked parties, I would attend more of them.
- If it rained tomorrow, people would dance in the street.
4. The Third Conditional (Past Unreal Conditional)
"Third conditional" is a pattern used to refer to hypothetical situations in a past time frame, generally counterfactual (or at least presented as counterfactual). Here the condition clause is in the past perfect, and the consequence is expressed using the conditional perfect.
- If you had called me, I would have come.
- Would he have succeeded if I had helped him?
5. The Mixed Conditional
"Mixed conditional" usually refers to a mixture of the second and third conditionals (the counterfactual patterns). Here either the condition or the consequence, but not both, has a past time reference.
#1. Present result of a past condition.
When the condition refers to the past, but the consequence to the present, the condition clause is in the past perfect (as with the third conditional), while the main clause is in the conditional mood as in the second conditional (i.e. simple conditional or conditional progressive, but not conditional perfect).
- If you had done your job properly, we wouldn't be in this mess now.
- If I hadn't married Kelly, I wouldn't be living in Scotland now.
#2. Past result of a present or continuing condition.
When the consequence refers to the past, but the condition is not expressed as being limited to the past, the condition clause is expressed as in the second conditional (past, but not past perfect), while the main clause is in the conditional perfect as in the third conditional.
- If we were soldiers, we wouldn't have done it like that.