“I can't remember if I sent it or not.”

You were planning to send an email to some friends inviting them to go out for dinner. Now you're not sure if you sent it. Uncertain, you say this.

I can't remember if I sent it or not.

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remember if (independent clause)

An "independent clause" is really just a sentence that's inside another sentence.
You can put any clause into this structure, as long as it has a yes/no answer:

remember if my mother went to school with her

You can put "or not" on the end of the phrase, or leave it off.

It's also common to use "whether" in place of "if".