As all these words denote the same meaning in the English language, don’t use any two words together in a sentence:
Suppose if the doctor operated on him in time, his life would be saved. ✗
Supposing = Used at the beginning of a sentence or clause to denote ‘what would happen if’:
Supposing (that) the would-be in-laws ask you why you left so lucrative job — what would be your reply?
However, we use be supposed to to express obligations and arrangements; it doesn’t mean for suggestions.
You are not supposed to stay here in the hospital after mid-night.
In conditional clauses, we use the present simple to refer to the future with if, in case and provided:
We’ll have the party in the house itself this Saturday if Father is out of town.
We often use be to + infinitive to express that something must happen first in the main clause, before the other thing can take place in the if-clause:
Anna Veith needs to improve her techniques, if she is to keep her hopes of gold alive at the next Olympics.
We use if…were + to-infinitive to say imaginary future situations (Unreal conditionals):
If the government funding were to become available, those neglected community hospitals would be better taken care of.
If she were to have a chance of winning gold in the next Olympics, she would need to train herself by a world-class expert.
When one situation is dependent on another situation or on a person, we use if it wasn’t/ weren’t for…:
If it wasn’t/ weren’t for Sharon, his acting career wouldn’t begin to take off.
If it hadn’t been for Sharon, his acting career wouldn’t have begun to take off.
Were it not for Sharon, his acting career wouldn’t begin to take off. (Formal & Literary)
Had it not been for Sharon, his acting career wouldn’t have begun to take off. (Formal & Literary)
We can use if…will when we talk about a result of something in the main clause.
Switch off the lights if it will help you to sleep.
However, using will twice in the same sentence is incorrect because adverbial clauses with when, if, provided, suppose, in case do not usually use the modal will. To indicate the future time within an if-clause, use the simple present:
The teacher will punish you if it will turn out that you stole the pen from Suzanne’s bag. ✗
We also use if…happen to, if…should, or if…should happen to to say about something which may be possible, but is unlikely to happen:
If you (should) happen to pass by my house, call in and have a cup of tea.
(Don’t use this pattern in unreal conditionals which express impossible events in the if-clause.)
If there was a blizzard, the school would be out before its schedule.
When two possibilities have been talked about or when we are not sure about something, we use whether/ if:
The Law makers couldn’t decide whether/ if it is worth trying to abstain from voting.
I’m not sure whether/ if Jane agrees with me on walking down the street in such late hours.
Provided (that) = If or only if:
Provided (that) the right teaching methods are available, we shall be able to solve the illiteracy problem in this village.
Some community services arrange for schoolchildren to work during their holidays and free time provided (that) proper government funding is available.
In case = if something happens:
Martha will bring a powerbank to the camping in case the battery runs low.
In case the car breaks down on your way home, ask someone for a ride.