Although "if" is the most common word used in conditional sentences, we can also use other words and phrases instead.

When can be used for an expected event:

For example:

• When Tom comes, we’ll start dinner.
  • Here, we expect Tom to arrive.

Compare this with:

• If Tom comes, we’ll play Monopoly.
  • Here, we are not 100% sure if Tom will come or not.

Unless can be used to give the idea of  "if not":

For example:

• Unless it rains, we’ll play golf this afternoon. (If it doesn't rain, we’ll play golf this afternoon).
• Tom should be here soon unless he missed his train. (Tom should be here soon if he didn't miss his train).

The following describe strong or strict conditions and give the meaning of "only if":

• As / so long as...,
• On condition that...,
• Provided (that).., 
• Providing...

For example:

• You can board the plane as long as you arrive at the gate 20 minutes before departure.
• Staff can take extended sick leave on condition that they have a doctor’s certificate.

Supposing can be used to give the idea of "what if...?"

For example:

• Supposing the boss had caught you using Facebook, what would have happened?

Otherwise is used to give the meaning of "or else"

For example:

• We have to leave soon, otherwise, we'll be late for the show.

"But for" means if it were not for / if it hadn’t been for (if this didn't exist)

For example:

• I wouldn’t be the success story I am but for your support. (I wouldn’t be the success story I am if I didn't have your support).
• But for the rain, we could have played tennis. (If it hadn’t been for the rain, we could have played tennis).

I hope this has helped to clarify the alternatives to the word "if" in conditional sentences. To learn more about the other types of conditionals, have a look at Common conditional forms and Mixed conditionals.