Other expressions instead of “if”

Unless  (if not)

• Unless he wasn’t so tired, he would help her.
• I’d like to come to see you at the weekend unless I don’t have to work.

Provided/Providing (means if, on condition that, but only if)

• He would help her provided he (that) wasn’t so tired.
• He’ll lend me the car providing I return it before the weekend.

As long as/So long as (means if, on condition that, but only if)

• Children can stay here as long as they keep quiet.
• You’ll be fine so long as you’re careful.

On condition that (means if, but only if)

• I’ll lend you the car on condition that you’re careful.
• You can go out on condition that you are back by midnight.

Supposing/Suppose (that) (what if/imagine if – we talk about the imaginary situation)

• Supposing (that) you won the lottery. What would you do?
• Suppose (that) he asks you to marry him, what will you say?
• Suppose (that) you failed the exam. What would you do?

In case (we get prepare for something that might happen) 

• I’ll take my umbrella in case it rains. (it’s just a preparation)
• I’ll give you the key in case I’m not at home. (it’s just a preparation)
• We’ll buy more wine if more people come.

Note

• I’ll go with you in case you need me. (He may need me or not, I’ll go in any condition.)
• I’ll go with you if you need me. (I’ll go only if he needs me.)