CONDITIONAL CLAUSES

A conditional sentence comprises a Principal Clause and a Subordinate Clause. In a conditional clause, the fulfilment of a condition depends on the fulfilment of the proposition of the sentence. It may be two types : open or real condition and hypothetical or unreal condition.

In an open condition, the condition may or may not be fulfilled, and depending on it, the proposition of the sentence may or may not be fulfilled; for example :

If she completes her paper, she will surely pass. (Here she may or may not complete her paper.)

If it is sunny, we can go on a picnic. (It may or may not be sunny.)

However, if the speaker thinks that the situation is unlikely or impossible, it becomes a hypothetical condition and Past Tense is used in such sentences; for example ;
If she completed her paper, she would surely pass. (It is unlikely that she would complete the paper.)

If it were sunny, we could go on a picnic. (It is almost unlikely or impossible to be sunny.)

If a hypothetical conditional sentence pertains to the past time, it would be framed as follows :

If she had completed her paper, she would have surely passed. (This is an unlikely or impossible situation pertaining to the past time.)

If it had been sunny, we could have gone on a picnic. (It is an unlikely or impossible situation of the past time.)