Conditional sentences

Conditional sentences are complex sentences that consist of two parts: a main clause describing an action and a subordinate clause stating a condition that must be met in order for the main action to take place. English conditional sentences require different verb forms depending on the time and reality of the main action and its condition: both can refer either to the present/future (and be real or unreal) or to the past (unreal).
1) A real present/future condition assumes a real main action: If I see him, I will run away. As you see, the Present Indefinite is used in the subordinate (future tenses are not allowed in a conditional clause!) and the Future Indefinite in the main clause.
2) An unreal present/future condition is expressed using the Past Indefinite Tense (the verb 'to be' has the form 'were' in all persons): If I saw him...(If I were you...). The predicate of the main sentence has the form 'would/could + verb'.

Example:

If I saw him now, I would run away. If I knew it before it started, I could do much better. 3) An unreal past condition is expressed using the Past Perfect Tense: If I had seen him... The predicate of the main sentence has the form 'would/could + have + Past Passive Participle of the verb'.

Example:

If I had seen him then, I would have run away. If I had known it on that day, I could have done much better.

Exception:

In some cases the use of tenses in the main and subordinate clause is independent of each other. For example, if a condition extends from past to present but the action is no longer possible, mixed rules can be applied: Past Indefinite in the subordinate and 'would/could + have + Past Passive Participle' in the main clause.

Example:

If I knew it (generally), I could have done better (in the past). One last remark concerns the fact that both the condition and the main action can be expressed by means other than a clause, or even just implied. The rules for the use of tenses in the remaining part of a sentence still apply.

Example:

At his appearance I would run away. Under other circumstances I could have done better. If only you had said that before!