Conditional Sentences

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. All conditional sentences contain a dependent clause and an independent clause. The dependent clause usually begins with if; it expresses a condition. The independent clause expresses a result of the condition. The if-clause is usually first, but the order of the clauses is usually not important.

Conditional Sentence Type 1

• Nature: Open condition, what is said in the condition is possible.
• Time: This condition refers either to present or to future time.

Form: if + Simple Present, will-Future

Example: If I find her address, I’ll send her an invitation.

Conditional Sentence Type 2

·         Nature: unreal (impossible) or improbable situations.

·         Time: present; the TENSE is past, but we are talking about the present, now.

Form: if + Simple Past, Conditional I (= would + Infinitive)

Example: If I found her address, I would send her an invitation.

Conditional Sentence Type 3

·         It is impossible that the condition will be fulfilled because it refers to the past.

Form: if + Past Perfect, Conditional II (= would + have + Past Participle)

Example: If I had found her address, I would have sent her an invitation.

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.

Note: Main clause and / or if clause might be negative. See Past Perfect and Conditional II on how to form negative sentences.

Example: If I hadn’t studied, I wouldn’t have passed my exams.

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 Ferrari. He would have loved to own a Ferrari, but he never had the money to buy one.

So far you have only learned the basic rules for Conditional Sentences. It depends on the context, however, which tense to use. So sometimes it's possible for example that in an IF Clause Type I another tense than Simple Present  is used, e.g. Present Progressive or Present Perfect. 

Remember!

1. The conditional construction does not normally use will or would in if-clauses. EXCEPTION: If will or would express willingness, as in requests, they can be used in if-clauses.

e.g. If you will come this way, the manager will see you now.
I would be grateful if you would give me a little help.
(= ± please, come this way; please, give me...)

2. For the second conditional, were replaces was:

If I were a rich man...

3. After if, we can either use "some(-one, -where...)" or "any(-one, -where...).

If I have some spare time next weekend....or :
If I have any spare time...

4. Instead of if not, we can use unless.

e.g. I'll be back tomorrow unless there is a plane strike.
He'll accept the job unless the salary is too low.

5.There is a "mixed type" as well, for the present results of an unreal condition in the past:

If + Past Perfect - would + inf.
If you had warned me [then], I would not be in prison [now].

https://www.ego4u.com/en/cram-up/grammar/conditional-sentences/exceptions

http://faculty.deanza.edu/flemingjohn/stories/storyReader$18

http://grammar.ccc.commnet.edu/grammar/conditional2.htm