That's interesting: I did not understand a word of the Turkish sentence (except for “lazım”), just typed what I heard, and it was accepted. Every time I think I considered three or four rules in a sentence it turns out that seven or nine (ten?) rules are needed… I guess I am simply too old to learn a new language. Sorry.
Never feel you are too old to learn a language. Turkish is different in Syntax. That's why it is interesting to learn it. I found a way to think in Turkish: Go the most important information first. Most likely it works. O zaman dans etmemiz lazım: O zaman= it is a "brick", take as it is, meaning "then". dance etemiz= this is the most importat part, the main information. Instead of saying "we dance" in Turkish is we say "we make, do dance", and lazim= it is necessary.
So it makes sense.
Sometimes it is difficult to find out how it is related all these, but here it is easy I think. Turkish is a very mathematical language, but different!
Good luck! :)
PS. It is like doing maths in an inverse way, as it is the reverse Polish notation: 3 4 + in RPN is the usual 3+4. Look at: https://en.wikipedia.org/wiki/Reverse_Polish_notation : )
The sound for this sentence is weird for a native speaker. I heard "O zamandan- setmemiz lazım" and typed "O zamandan seçmemiz lazım", which means "We need to choose since then" (or you could say "O zamandan İTİBAREN seçmemiz lazım". (-dan itibaren = beginning (from)) (itibar etmek = to rely on, to approve, to consider or to honor) (I know, a huge variant of meanings)