Learn Turkish: Tezgahtaki antikaya bakıyorum. - I am looking at the antique on the stall.

The suffix meaning 'on' or 'in' usually starts with a 'd' (-da or -de). But because tezgah ends in a hard consonant (the letter h), the 'd' changes to a 't' to match it. This makes it tezgahta. Then we add -ki to turn it into an adjective: the one on the stall.

If I take away -ki, what does Tezgahta antikaya bakıyorum mean?

It changes the meaning from 'I am looking at the antique that is on the stall' to 'I am on the stall, looking at the antique.' The -ki suffix connects the location directly to the antique. Without it, tezgahta just tells us where you, the speaker, are standing!

Does the -ki suffix ever change to fit vowel harmony?

Usually, no! Unlike most Turkish suffixes you have learned, -ki is a rebel. It almost always stays exactly as -ki, no matter what vowels come before it. So even after the thick 'a' in tezgahta, it does not change to a thick vowel.

Why does antika have the -ya ending here?