Learn English – “here’s it” versus “here it’s”

‘An A-Z of English Grammar and Usage’ by Leech and others suggests the following:

With be, use the contraction + not (e.g. That’s not right).

With have and modal auxiliaries, use the verb + n’t option, e.g. hasn’t, can’t.

The authors go on to say that forms such as She isn’t hungry, as opposed to She’s not hungry, are less common, and that forms such as I’ve not met him, as opposed to I haven’t met him, are much less common.

The authors don’t say on what basis they make their recommendations, but they are likely to be based on frequency of use. There doesn’t seem to be any difference of meaning or emphasis. I would just add myself that a contraction such as I’ve not met him sounds more formal, perhaps because of its lower frequency.

Both are used and both are acceptable, though you are most likely to hear "that's" rather than "that is/was".

However, "is" cannot be used if the reason, per se, happened in the past.

'Remember when you got an F?'

'Yes. That was because I couldn't find my homework because you hid it.'