Breakdown of Musimy udowodnić ten fakt, dlatego że ktoś tu był.
być
to be
tu
here
ten
this (m)
musieć
to have to
Questions & Answers about Musimy udowodnić ten fakt, dlatego że ktoś tu był.
Can I use just dlatego without że in this sentence?
No, because that would completely change the meaning! Dlatego że means "because" (giving a reason). If you just used dlatego, it would mean "therefore" or "that's why" (giving a result). Saying "We must prove this fact, therefore someone was here" wouldn't make logical sense in this context.
Is dlatego że different from bo?
They both mean "because", but dlatego że sounds a bit more formal and heavily emphasizes the specific reason. Bo is much more common in everyday, casual speech. You could perfectly well say ...bo ktoś tu był here, but dlatego że matches the serious tone of having to prove a fakt (fact).
Udowodnić (to prove) is an action done to an object. Why doesn't ten fakt change its ending for the accusative case?
It actually is in the accusative (direct object) case! However, for masculine inanimate nouns like fakt, the accusative form looks exactly the same as the basic dictionary form. The word for "this" (ten) also stays exactly the same for masculine inanimate objects.
"Someone" (ktoś) could be a man or a woman. Why does the verb był (was) have a masculine ending?
In Polish, the pronoun ktoś (someone) is always treated as grammatically masculine, regardless of who you are actually talking about or if their gender is unknown. That is why it must take the masculine past tense form był.