'Tá mé or 'Is mé'?
I've never seen 'Is mé' before joining the Duolingo course.
Some things are expressed with bí, e.g. Tá mé fuar, and some things are expressed with is, e.g. Is ciúb oighir mé. A rough division is that statements of identification or classification use is, and statements of description or existence use bí.
Since you’ve done some of a Portuguese course here, an analogous (but not identical) division in Portuguese is that of ser vs. estar ; is is like ser, and bí is like estar.
As a general rule you would only use "Is mé" if you were asked using An Chopail. So if someone asked you "An tusa an múinteoir?", you might say "Is me an múinteoir" but if someone asked using the question form of bí "an bhfuil tú ag múineadh?" then you would reply using bí "Tá mé ag múineadh. Sorry if this explanation is vague!
I'm obviously still really new to Gaeilge, but is "An tusa an múinteoir?" a correct way to word the question?
I tend to learn languages by "feel" and "sound." To my ear it should be something more like "An múinteoir thú?"
Hopefully someone will chime in here.
An tusa an múinteoir? - "Are you the teacher?"
An múinteoir thú? - "Are you a teacher?"
I found this page from Cork Irish to be of great value: https://corkirish.wordpress.com/2011/09/19/is-and-ta-their-meaning-their-difference/
In short, use "Is" when predicating substance. So: "Is clog adhmadh é" or "Is duine mé"
And use "tá" when predicating mode: "Tá sé ag siúl" Manner: "Tá sé tapa" Place: "Tá sé i Corcaigh" Or quality: "Tá sé cruaidh"
There is more to it though, so I highly recommend you to read the article.
"Is clog adhmadh é" or "Is duine mé"
Noun and pronoun - use the copula.
And use "tá" when predicating mode: "Tá sé ag siúl"
Noun and progressive verb (formed by a preposition and the verbal noun)
Manner: "Tá sé tapa"
Noun and adjective
Place: "Tá sé i Corcaigh"
Noun and preposition
Or quality: "Tá sé cruaidh"
Noun and adjective