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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.
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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.
Comments  
'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.