Learn Hardcore Polish: Rzeka, którą widzieliśmy z mostu, była wysoka po listopadowym deszczu. - The river that we saw from the bridge was high after the November rain.

Którą is the accusative feminine singular form of the relative pronoun który. It refers back to rzeka (a feminine noun) and functions as the direct object of widzieliśmy.

Why is mostu in the genitive case?

The preposition z (“from”) requires the genitive case. Therefore most becomes mostu, giving z mostu (“from the bridge”).

What role do the commas around którą widzieliśmy z mostu play?

They set off a non-restrictive (parenthetical) relative clause, adding extra information about rzeka. Without them, the clause would be essential to identify which river you mean.

How is the past tense widzieliśmy formed, and why is widzieć imperfective here?

Widzieliśmy is the 1st person plural past form of widzieć.
Widzieć is imperfective, so it describes the action of seeing as a completed event without focusing on its result (we saw it, without implying we got to a new “seen” state).

Why is była wysoka used to describe a river?

Była is the past tense, feminine singular of być (“to be”), agreeing with rzeka.
Wysoka is a predicate adjective matching in gender and number. Here wysoka means “high” in the sense of a raised water level (not tall).

What does po listopadowym deszczu mean, and what case does po require?

Po means “after” and indicates the time/cause: “after the November rain.”
Po takes the locative in time expressions, so listopadowy → listopadowym, and deszcz → deszczu.

Can you move the relative clause którą widzieliśmy z mostu elsewhere in the sentence?

In Polish, the relative clause normally follows immediately after its noun (rzeka). Placing it elsewhere can confuse the linkage between the clause and rzeka, so it’s best to keep that position.