Conditional Sentences are also known as Conditional
Clauses or If Clauses. They are used to express that the action in the main
clause (without if) can only take place if a certain condition (in the clause
with if) is fulfilled. There are three types of Conditional Sentences.
IF Clause Type 1
Form
IF+ Simple Present, will-Future
Example: If I find her address, I will send her an
invitation.
The main clause can also be at the beginning of the
sentence. In this case, don't use a comma.
Example: I will send her an invitation if I find her
address.
Use
Conditional Sentences Type I refer to the future. An
action in the future will only happen if a certain condition is fulfilled by
that time. We don't know for sure whether the condition actually will be
fulfilled or not, but the conditions seems rather realistic – so we think it is
likely to happen.
Example: If I find her address, I’ll send her an
invitation.
I want to send an invitation to a friend. I just have
to find her address. I am quite sure, however, that I will find it.
Example: If John has the money, he will buy a Ferrari.
I know John very well and I know that he earns a lot
of money and that he loves Ferraris. So I think it is very likely that sooner
or later he will have the money to buy a Ferrari.
IF Clause Type 2
Form
IF + Simple Past, main clause with Conditional I (= would
+ Infinitive)
Example: If I found her address, I would send her an
invitation.
The main clause can also be at the beginning of the
sentence. In this case, don't use a comma.
Example: I would send her an invitation if I found her
address.
Were instead of Was
In IF Clauses Type II, we usually use ‚were‘ – even if
the pronoun is I, he, she or it –.
Example: If I were you, I would not do this.
Use
Conditional Sentences Type II refer to situations in
the present. An action could happen if the present situation were different. I
don't really expect the situation to change, however. I just imagine „what
would happen if …“
Example: If I found her address, I would send her an
invitation.
I would like to send an invitation to a friend. I have
looked everywhere for her address, but I cannot find it. So now I think it is
rather unlikely that I will eventually find her address.
Example: If John had the money, he would buy a
Ferrari.
I know John very well and I know that he doesn't have
much money, but he loves Ferraris. He would like to own a Ferrari (in his
dreams). But I think it is very unlikely that he will have the money to buy one
in the near future.
IF Clause Type 3
Form
IF + Past Perfect, main clause with Conditional II
Example: If I had found her address, I would have sent
her an invitation.
The main clause can also be at the beginning of the
sentence. In this case, don't use a comma.
Example: I would have sent her an invitation if I had
found her address.
Use
Conditional Sentences Type III refer to situations in
the past. An action could have happened in the past if a certain condition had
been fulfilled. Things were different then, however. We just imagine, what
would have happened if the situation had been fulfilled.
Example: If I had found her address, I would have sent
her an invitation.
Sometime in the past, I wanted to send an invitation
to a friend. I didn't find her address, however. So in the end I didn't send
her an invitation.
Example: If John had had the money, he would have
bought a Ferrari.
I knew John very well and I know that he never had
much money, but he loved Ferraris. He would have loved to own a Ferrari, but he
never had the money to buy one.
Source :
http://www.ego4u.com/en/cram-up/grammar/conditional-sentences