Definition
of a conditional clause
A conditional clause is a type of
subordinate clause, most commonly introduced by the conjunction if or unless.
Like most subordinate clauses introduced by a conjunction, the
conditional clause can either go before the main clause, or after it.
This usually refers to a future event
which is conditional on another future event.
The verb of the main clause is in the future tense with "will" (or sometimes another modal). The verb of the conditional clause is in the simple present tense.
The verb of the main clause is in the future tense with "will" (or sometimes another modal). The verb of the conditional clause is in the simple present tense.
Example :
1. If you eat too much, you'll get fatter.
2.
If I go to London, I can visit the British Museum.
3. If everyone works fast, we'll finish in time.
This refers to a possible future
situation which depends on on another possible future situation. The verb of
the main clause uses the present conditional tense (would +
infinitive, or could+infinitive), The verb of the conditional clause normally
uses the present subjunctive or preterite (these two tenses are identical
except with to be). Occasionally, the conditional aspect of the statement
can be emphasised by using the form were + to + infinitive.
Example :
1.
If I went to London, I would / could
visit the British Museum.
2. If you visited Scotland, you could see Edinburgh
Castle.
3. Unless the directors increased sales, we'd
have to close this shop.
This refers to a situation which an
event might have taken place, but did not, because a condition
was not fulfilled. The verb of the main clause
goes in the past conditional (would have + past participle). The verb of
the conditional clause goes in the past perfect (had + past
participle).
Example :
1.
If you had eaten too much, you'd (you would) have got fatter.
2. If I had gone
to London, I could have visited the British Museum (but I didn't).
3. If you had
visited Scotland, you could have visited Edinburgh Castle (but you didn't).
Source : http://linguapress.com/grammar/conditionals.htm