As phoenix7777777 pointed out, it’s not tha but cha.
Tha pronounced /ha/ is the present-tense independent form of bi, the substantive verb.
Cha pronounced /xa/ is the negative particle not, and also the negative form of the copula is (generally the copula disappears after particles, so the question form of the copula is the same as the question particle too – an, do you like…? being an toil leat…?).
They both do sound similar, but they are not the same word and have very different meaning. ;-)