Noun Clauses with “whether or if”

Noun clauses with “whether” or “if” are used to introduce indirect questions or express doubt or uncertainty.

They introduce a yes/no question or condition and act as the subject, object, or complement of a verb.

as subjects

When they are used as subjects only “whether” or “whether or not” is possible. We cannot use “if” to introduce a subject noun clause.

Here are some examples:

(Will he be promoted? It is still uncertain.) Whether (or not) he will be promoted is still uncertain. (Should we invest in stocks? It is a decision we need to make.) Whether (or not) we should invest in stocks is a decision we need to make. (Will it rain tomorrow? It may change our plans.) Whether (or not) it will rain tomorrow may change our plans. Alternatively, or not can be used at the end of the noun clause. Whether he will be promoted or not is still uncertain. Whether we should invest in stocks or not is a decision we need to make. Whether it will rain tomorrow or not may change our plans. The noun clauses here introduce questions and express uncertainty.

as objects

Both “whether (or not)” and “if / if ….or not” can be used as object noun clauses.

Look at this example:

(Has she called the office?) Here are all the alternatives with whether (or not) and if / if . or not We don't know if she's called the office. We don't know if she's called the office or not. We don't know whether she's called the offfice. We don't know whether she's called the offfice or not. We don't know whether or not she's called the office. Be careful! When "if" is used with "or not"; the use of "or not" is only possible at the end. I'm not sure if  she is here or not. (Correct!) I'm not sure if or not she is here . (Wrong!)

Here is another example:

(Mary: Should we go out for dinner?) Mary asked if we should go out for dinner. Mary asked if we should go out for dinner or not. Mary asked whether we should go out for dinner. Mary asked whether we should go out for dinner or not. Mary asked whether or not we should go out for dinner.

as complements

When used as complements only whether is used. (“if” is possible in informal usage.)

The question is whether we should trust him. The issue is whether she is qualified for the position. Tom is unsure whether his wife will like the gift.

Note:

Just like noun clauses with question words, "whether" can be followed by an infinitive. They're not sure whether they should call the police (or not). They're not sure whether to call the police (or not). We use "whether" if this type of noun clause follows a preposition. We quarreled about whether we needed to buy a new car (or not).