"It is a poor family."

Translation: To biedna rodzina.

[amnae]

Why is 'Jest biedną rodziną' wrong?

[mihxal]

"Jest biedną rodziną" sounds incomplete. There should be some additional noun at the beginning and it would mean "Is a poor family".

[Gerardd88]

Generally, don't use this 'jest + instrumental' form for the third person without the subject. The subject is mostly default for Jestem, jesteś, jesteśmy, jesteście (you don't need to say ja jestem etc) but usually not for jest, są. This to construction sounds better in most cases.

[immery]

I disagree. I think this comes from the fact, that rodzina is a specific noun,

so in this case I would say (oni) "są biedną rodziną"= they are a poor family.

sentences like
jest nauczycielem = he is a teacher, are ok.

[Gerardd88]

You're right but what I meant was rather that, to me, sentences like "On jest nauczycielem" sound more natural or better with the subject than without it. The same is the case for your other example "(Oni) są biedną rodziną".

I wanted to stress that you can almost always easily use this construction with ja, ty, my, wy, while it can be often tricky with on, ona, ono, oni (another example: To jest pies vs Jest psem). Also as I said, sentences using to (jest) construction tend to sound better so I think it's better for the beginners not to use this construction in those contexts.

[immery]

I think it is this is vs he/she/it is. Those are different sentences ( In Polish).

This is = to jest is an introduction

he/she/it=(on/ona/ono) jest
is is a further description.

It might be personal preference or something, but it really frustrates me when I have to write "on jest zmęczony", in the English (from PL) course.