CONDITIONAL SENTENCES:

There are many ways to make sentences with IF. Most tenses are possible.

Oil floats if you pour it on water.
If I won a lottery, I would travel around the world.
If you had worked harder last year you would have passed your exam.

There are three types of conditional sentences
1st Type: they refer to 'real, possible' situations.
                                
If+ present simple+ * future tense: (possible situations for the future)
If+ present simple +* simple present ( Universal truths)
If + present simple+ *imperative   
If+ present simple+ * modal verb

If she invites me, I will probably go to the party.
Oil floats if you pour it on water
If you are so tired; go to bed.
If I have time, I might go shopping.


2nd Type: they are used to talk about unreal or hypothetical situations:

If + past simple+ conditional tense( would+ bare infinitive).

If I had longer holidays I would be perfectly happy.
I would like you better if you were quieter
If I were you, I would invite her 

3rd Type: When we want to talk about things that didn't happen in the past ( imagining what would have happened if things had been different...)

If+ Past perfect+ perfect conditional ( would have+ past participle)

If you had asked me, I would have told you the whole story.
I would have gone to the party If she had invited me
If we hadn't left so early, we would have missed the train. 


CONDITIONAL SENTENCES are often introduced by IF or UNLESS(=if not), but the following expresions can also be used instead of IF: providing, provided (that), as long as, in case, even if, only if...
Unless you reserve a ticket, you won't get a seat
You will get a seat as long as / providing/ only if you reserve a ticket .

WE CAN OMIT" IF" ; when we omit "IF" SHOULD, WERE, HAD ( past perfect) come before the subject. 


(1st type)
*  If he turns up, tell him to wait for me  = 
 SHOULD he turn up, tell him to wait for me.


(2nd type)
*  If I were you, I would speak to her  =    
Were I you, I would speak to her 


(3rd type)
 * If he had known, he would have told us  =   
Had he known, he would have told us.
BUT FOR ( + NOUN GROUP)= ( IF IT HADN'T BEEN FOR...)
Ex: But for the weather; we would have gone to the beach
( If it hadn't been for the weather, we would have gone to the beach)

But for my parents; I would have missed the train 
(If it hadn't been for  my parents ...I would have missed the train...)