We use the conjunction unless to mean 'except if'. The clause which follows unless is a subordinate clause (sc): it needs a main clause (mc) to make a complete sentence. When unless comes before the main clause, we use a comma: Unless [SC]it rains, [MC]we'll go for a picnic by the river tomorrow.

How do you use unless?

Often when we are talking about present situations, we use unless instead of if...not. Unless means except if or simply it means if...not. Both of these examples have the same meaning and refer to the present time. "You can't go on vacation unless you save some money."

What is the difference between if and unless?

As you probably already know, “if” is conditional. It is used to describe a possible situation, and what would happen under the conditions of that situation. “Unless” is also conditional, but it is effectively “if” with an extra exclusion or negative qualifier added. Unless basically means “except if” or “if… not”.