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.
"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.
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.