Learn English – How to say that the English isn’t very good

‘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.

This is one of the basic meanings of 'that':

that adverb /ðæt/ B2. as much as suggested

Thus, one would say something like

On the train everyone went to bed around 20:00, but I wasn't used to sleeping that early.

That (and this), so, and how can all be used as adverbs of degree, but only that or so would be used here; how is typically used when the degree is unknown, e.g. I didn't know how early we would sleep.