Question

If means on (the) condition that, provided (that), providing (that), presuming (that), supposing (that), assuming (that), as long as...

 

Unless means something similar to ‘if … not’ or ‘except if’.

 

The verb forms in the examples are similar to sentences with if: we use the present simple in the unless-clause and shall, should, will, would, can, could, may or might in the main clause.

 

Examples:

"You can't go on vacation unless you save some money."
"If you don't save some money, you can't go on vacation."

"You will feel cold if you don't wear a coat."
"You will feel cold unless you wear a coat."

 

Both if and unless are used to introduce conditional sentences.

 

 

Warnings:

We don’t use unless for impossible conditions:

If the government had not raised food prices, there would not have been so many protests.

Not: Unless the government had raised food prices …

We don’t use unless and if together:

We’ll go to the coast tomorrow unless it rains.

Not: We’ll go to the coast tomorrow unless if it rains.