adj plus if-clause

Question

Can if-clause be used as a noun clause after an adj? Does the sentence "She was undecided if she should accept his offer" sound natural to you? Or should I say "She was undecided whether she should accept his offer"?

Thank you very much!

Answer 1

She was undecided if she should accept his offer.

is just a statement of fact, and is OK as is.

She was undecided whether she should accept his offer or not.

Using whether usually indicates a choice of two alternatives. Many times you will also find the phrase *whether or not". See the definition of whether sense 2.
You could also say:

She was undecided whether or not she should accept his offer.

Answer 2

There's no specific grammatical rule governing how we link undecided to immediately-following text clarifying exactly what the indecision relates to. But consider these Google Books results...

I was undecided whether I [should do this or that] (2420 hits)
I was undecided if I... (72 hits)
I was undecided as to whether I... (115 hits)
I was undecided as to if I... (1 hit)
he was undecided whether he... (2840 hits)
he was undecided as to whether he... (1410 hits)

Clearly native speakers overwhelmingly prefer whether over if in such constructions. Personally, I'm in that "significant minority" who also prefer to include as to - but that's really style, rather than grammar.

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In some contexts, there can be a semantic distinction. Paraphrasing Grammar Girl's examples,...

1: Squiggly was undecided whether he would go out on Friday or Saturday.
2: Squiggly was undecided if he would go out on Friday or Saturday.

As indicated, I don't much like #2 anyway. But semantically most people would say there's no doubt #1 implies Squiggly definitely will go out - he just hasn't yet made up his mind which day. Whereas with #2 it's possible that he's thinking of staying in this weekend.

Note that this does not imply you "should" use if whenever there's only one possible choice (with the unstated alternative not that choice) - it's just that you can. But it's worth being aware of the fact that using whether instead of if can avoid potential ambiguity in some contexts.