"Feasgar math. Is mise Anna."
Translation:Good afternoon. I am Anna.
Yes, a really fundamental one.
"Tha" is only for describing things (e.g. with adjectives or prepositions)
"Is" (the copula verb, often contracted to 'S) must be used when defining one noun as another (in this case a pronoun as a named individual).
So, e.g. "Tha mi Anna" is completely ungrammatical.