Is there any textual usage difference between words like "I've" and "I have", or is it just an accent thing?
I've finished my homework.
I have finished my homework.
It's a beautiful day.
It is a beautiful day.
It is just the contracted form.
Contracted forms are not used when verb has its own meaning.
— I have my homework. (have stands for possession.)
I've my homework.
— I have finished my homework. (have is an auxiliary.)
They are not used in short answers:
— Have you done your homework?
— Yes, I have.
Also, when writing formal stuff, you normally use non-contracted forms. For instance, in an essay.
There's a difference in spoken English when you stress the non-contracted form. Compare:
— I've finished my homework.
— I have finished my homework.
(Emphasis when speaking.)
You can't use pronoun-auxiliary contractions if they're final in a sentence
(so they can't be used in tag questions, for instance).
In general, if it's unstressed -- which is the norm, and a good reason for contraction -- a pronoun subject will be contracted with an auxiliary if there is one. The more stress the pronoun has, the less likely it is to be contracted with an auxiliary. The faster you're talking, the more likely it is. English speakers make decisions like this every time they open their mouths, automatically.
Since it's hard to stress pronouns (the reason we use pronouns is to avoid extra stressed syllables), it's almost universal to contract pronoun subjects and auxiliary verbs in English,
unless the auxiliary verb is already contracted with another word, like isn't.
In writing, of course, there is no good way to represent contractions.
which are problematic -- they don't represent English, just typography.
They're, their, and there, for instance, are pronounced identically in English;
however, English speakers never feel confused about what they mean.
Only about how to spell them.
've is the abbreviation of have. The same thing but different in writing in a long or short-term which is up to you to choose which way you write. But it's better to do it when after 'have' there is a verb:
But when you talk about possessions. For example:
I have a blue jacket.
Its not that necessary.
And don't mix up between abbreviating is to 's And has to 's
She's been to London. → She has been to London.
She's beautiful. → She is beautiful.