Learn Polish: Muzyk przerobił całą aranżację, która była nudna. - The musician reworked the whole arrangement, which was boring.

The base verb is robić, which means "to do" or "to make". The prefix prze- here adds the sense of re-doing or altering something that already exists. Together, przerobić means "to rework", "to remake", or "to alter".

Why does aranżację end in -ę here?

It is the direct object of the verb przerobił (reworked), so it takes the accusative case. The base word is aranżacja (arrangement). Like most feminine nouns ending in -a, the ending changes to -ę in the accusative. The adjective attached to it also changes to match, becoming całą.

Why is która in the nominative case when it refers back to aranżację, which is accusative?

A relative pronoun like która (which) matches the gender of the noun it refers to (feminine, for aranżacja). However, its case depends on its job in its own part of the sentence. In the phrase która była nudna (which was boring), "which" is the subject, so it must be in the nominative case.

Why is it była nudna (nominative) instead of była nudną (instrumental)? I thought we use the instrumental case after the verb "to be".