Language/English/Grammar/When-use-If-or-Whether

When to use "whether" or "if" in English?

We must use "if" in a sentence to the conditional (A sentence that can only be realized under certain conditions):

Bob didn’t know if Amanda was going away.

You must use "if" when you show that there are only two possible outcomes:

Bob didn’t know whether Amanda was staying in the city or going to the country side for the weekend.

You can use either "whether" or "if" when: ... one of the issues is induced but understood, as in an indirect question:

Bob didn’t know whether Amanda was going away on Saturday.

Bob didn’t know if Amanda was going away on Saturday.

In this example, the term "or not" is induced. So, Amanda might not leave on Saturday. Since the second option does not need to be stated, you can use either "if" or "if".

Only "whether" can be used when:

... we find a verb in the infinitive with "to":

Bob didn’t know whether to invite Amanda to the party.

... we find a preposition:

Bob wanted to have a conversation with Amanda about whether she was away for the weekend.

... the word "or" is used:

Bob didn’t know whether to call or text to find out about her plans.

... when the part of the sentence beginning with the conditional is the subject of the sentence:

Whether Amanda could come or not, Bob was still going to throw the party, even if it was for Amanda's birthday.