"We are really healthy."
Translation:Tha sinn gu math fallain.
Learner myself, but I'll try --
First, sinne is the emphatic form of sinn. They both mean "we", but sinne is "WE". Emphatic forms of pronouns are used in some of the grammar patterns associated with the "Is e" forms of "to be." Emphatic forms are also used generally to emphasise that point in a sentence.
With respect to starting a sentence with either "Is" or "Tha" ---
Tha (tha, chan eil, a bheil?, nach eil?) -and- Is (Is e, chan e, an e?, nach e?) are two different forms of the verb "to be" and use different grammar patterns.
Use "tha + pronoun/noun + adjective" when describing or saying something about the pronoun/noun that is the subject.
Use "Is" forms when writing a sentence or question that identifies the noun/pronoun that is the subject as being some other thing (noun).
Is sinne na tidsearan = we are the teachers (na here is the plural article)
Tha sinne glè thoilichte = we are very happy
But "Is sinne glè thoilichte" would not be grammatically correct. I think people would understand what you were saying though.
[There is a grammar pattern that Duolingo hasn't covered at this point in the course that would allow "Tha sinne na tidsearan" and it means "we are teachers" BUT "na" in this sentence is a form of "ann an" meaning "in our". (And there is no article.)]
TAIC (taic.me.uk) explains more about "is e" patterns in Lesson 19 The Assertive Verb.
And there are some examples in "Is Ann" by Moffat-Pender, but it is from 1930 so spellings are different. ('S e patterns and 'S ann patterns are covered.)
Gaelic in 12 Weeks (OMaolalaigh) covers "Is e" patterns in Lesson 2 (p8+), and Lesson 3, section 21 (p23+).
Is forms are also covered under Phrases 2 just after the first castle in the course notes: https://duome.eu/tips/en/gd