It's not important if it rains or not.
Does this sentence sound odd to you because of using 'if' instead of using 'whether'?
It's not important if it rains or not.
Does this sentence sound odd to you because of using 'if' instead of using 'whether'?
It's not important if it rains or not.
There's nithing wrong with the sentence; it's grammatical.
You can use either whether or if with "or not" in end position, with no difference in meaning. However, the use of whether is far more common and idiomatic in the sentence.
You can also use "or not" immmediately after whether as follows:
It's not important whether or not it rains.