I am curious about the colloquial versions of "You", "your" etc.
a) I understand "ya" as object can be always used: "I'm telling ya" etc. Can it be used also as subject? I think so because I found some examples. But can it be really always be used as subject? I've thought of these examples: "Are ya (or ar'ya?) sure about that?" "Ya're really not that tall." Do you think they are correct, although obviously very informal?
b) is there a corresponding possessive form (your/yours)?
Thank you.
As a fast way of saying "you" it shows up in "ya think?" (A saracstic way to say "That's obvious.") and in your "Ya sure about that?" which also has a sarcastic tone (to mean "that's a stupid choice.")

If I am genuinely asking someone if they were certain, I am much more likely to say "you" than "ya."

I would not try to contract it. "Ya're" looks and sound ridiculous. "Ya really ain't all that tall" could be dialogue in a novel if you wanted to show a certain social class for the character, but otherwise, never write this way.

If you're learning English, I would not TRY to use them at all. Stick with "you" and if your speech becomes natural enough that "ya" shows up, good, but don't attempt it.
'Ya' is not a word; it is simply a phonetic description of the pronunciation of 'yes' by many native speakers in casual conversation.
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 BarbaraPA's reply was promoted to an answer.
Yes I was thinking of these forms as a joke of course I would never use them. God to know that they can have a sarcastic nuance though. Thank you.