When "has" is functioning as a main verb, it isn't contracted.
It has four legs
The verb is "has". That is the main verb. The contraction is not possible.
It has been moved.
The verb is "has been moved", and "has" is an auxiliary. The contraction is possible.
It has got four legs
The verb is "has got", ...
It's could be read as It has or It is. Hence -
It's a leg (It has a leg) or (It is a leg). This is ambiguous, so you must make the meaning clear with It has.
It's been moved (It has been moved) but not (It is been moved). No ambiguity, so it is correct to say It's.
One way to ask a question in English is to use subject-auxiliary inversion, for example
It should. - statement
Should it? - question
So it is reasonable to expect that the same would apply to questions phrased in the negative:
It should not.
Should it not?
In format written English, this is the correct way to proceed, but in spoken English it is normal ...
Verbally, people do this all the time, you're right about that.
I think whether it would (it's?) allowed in written speech depends a little on the thing being written. I wouldn't want to use that contraction in anything that was at all formal, but writing a personal letter to someone or relating dialog in a story of some sort, I would use the apostrophe that ...
Yes, "they aren't" and "they're not" mean the same thing, and "he isn't" and "he's not" also mean the same. There is some regional variation in terms of which way of saying things is more common. There may also be a subtle difference of emphasis, but it's difficult to pin down - perhaps in a noisy environment, "...
You can contract "is" and "has" when they are auxiliary verbs to 's
You can contact "are" to "'re" and "am" to "'m"
You can contract "have" when it is an auxiliary verb to 've
You can contract "had" and "would" (auxiliary) to 'd.
You can contract "will" and &...
There are standard contractions and there are contractions used by writers in dialogue:
So, contractions and tags like the ones below, are standard:
He must have left early, mustn't he? [standard]
He mustn't have liked them much, must he? [the tag here is less usual]
They speak Hindi, don't they? [standard]
They should have done the work better, shouldn't ...
Most informal contractions like these will be written down when the writer is conveying spoken English. Then anything the reader can understand is acceptable.
In your example you want
but I don't see how that's relevant
The way you wrote it the contracted "is" is in the wrong place.
For clarity, "pregnant with..." usually refers to the unborn baby. If referring to the other parent, we say "pregnant by...".
Assuming you understand this, "I'm pregnant with nobody" isn't good grammar, strictly speaking. You would simply say "I'm not pregnant".
However, there can be contexts in which we may use that ...
Yes, there's a little difference.
...I was thinking you missed my message
means that I had come to the conclusion that you had not gotten my message.
... I was thinking you might have missed my message
means I thought there was a possibility that you didn't get the message. I had not come to a conclusion.
In use, a speaker could say either phrase and ...
In conversation, more than 70% each of uses of "be" and "will" are contracted, about 55% of uses of "have", and only about 15% of uses of "would". In fiction, the figures are about 45%, 50%, 10%, and 5% respectively. In news reporting, they're about 10%, 5%, 5%, and 1%. In academic writing they're about 1% each, or ...
A distinction should be made between many items on a single to-do list or many to-do lists. Additionally, to-do is the adjective describing the type of list. The noun "list" is often understood and omitted.
If you want to pluralise or emphasize the number of tasks then I would suggest something like:
I have many tasks (or items) on my to-do list ...