When I speaking English I think in my mind that's easier to pronounce the word NOT separately from the verb to be like are or is for example instead of I say you aren't most if the times I'd say you're not or it's not so to me it's preferable use I've not it's clearer I don't know why but I think I'm not saying the negative form when I use these contractions isn't, aren't or haven't More @Wikipedia
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