"We are professional."
The vast majority of predicative adjectives are stated using bí rather than is.
Is there a reason muid can't follow proifisiúnta in this sentence?
Yes; using the analytic verb form, muid would be the sentence’s subject, proifisiúnta would remain a predicative adjective, and the subject would still precede the adjective.
Thanks! I'm having a lot of difficulty with when to use is and tá, but I think I'm (slowly) getting the hang of it.
If you’ve ever studied Spanish (or Portuguese), the difference between ser and estar is akin to the difference between Irish is and bí. (Tá is a conjugation of bí.)
I can't respond directly to your post, sciling, but I have heard that about Spanish (I didn't know that it was the same with Portuguese), but I have never learned either of them. I've kind of put off learning the copula (right?) but now might be a good time to tackle it.