CONDITION:

When writing or speaking we often wish to show that one event depends on another in some way:
If the weather was fine, Maud liked to walk in Hyde Park.
One statement, Maud liked to walk in Hyde Park, is conditional upon the other the weather was fine.
Conditional clauses are usually introduced by either if or unless.
They can express a number of different meanings.
Common events
They can state general truths, such as:
If water penetrates window sills, doors, or their frames, the result is wet rot.
In sentences like this the verb is in the present tense. It is also possible to use the past tense to describe general truths about the past:
If the weather was fine, Maud liked to walk in Hyde Park.
Possible events
Conditional clauses can describe situations which have not yet happened, but are possible:
If it comes to court, you two can testify.
Here both verbs are in the present tense. Similar sentences can be constructed using unless:
Policemen don't find bodies unless they are sent to look for them or unless someone else has found them first.
Here unless has the meaning of if…not…:
Policemen don't find bodies if they aren't sent to look for them or if someone else hasn't found them first.
Future events
Very often conditional clauses speculate about events in the future. Such clauses can be open or closed. In an open conditional the speaker expresses no opinion about whether the future event is likely to happen or not:
If they succeed in that, Germany's economy and its workers will be better off.
(The writer has no opinion of whether they will succeed or not.) In a closed condition the writer makes it clear that the future event is more or less unlikely:
If they were successful at this stage, they would then have to find the fee.
(But they are not likely to be successful.)
Past events
Conditional clauses can also be used to speculate about how things might have turned out in the past:
If they had been her own children, she would have used them differently.
But they weren't her own children, so she treated them as she did. The condition cannot be fulfilled because it is impossible.
Clauses that are not introduced by a conjunction
It is possible to construct conditional clauses that do not begin with if or unless. The commonest way of doing this is to begin the clause with one of these words:
were  should  had
For example:
Were I to own a new BMW car, another ten microcomputers would be at my command, so their advertisements claim.
Should you succeed in becoming a planner, you would be helping to create these parameters.
Had I been in a vehicle, I could have gone back, but on foot it was not worth risking the wasted energy.

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