Learn Dutch: Zij zingt altijd heel zuiver. - She always sings perfectly in tune.

The word zuiver literally means 'pure' or 'clean'. However, in the context of music or singing, it is the standard Dutch term for being 'in tune'. Adding heel (very) makes it 'very pure', which translates naturally to 'perfectly in tune' in English.

Could I use ze instead of zij in this sentence?

Yes, you absolutely can. Zij is the emphasized form of 'she', which you would use if you want to contrast her with someone else (for example, if someone else sings out of tune). If you are just making a general statement, the unstressed form ze is actually more common in spoken Dutch.

Why does the time word altijd come before heel zuiver?

Dutch sentence structure typically follows the Time-Manner-Place rule for adverbs. The adverb of time (altijd) comes directly after the conjugated verb, followed by the adverb of manner (heel zuiver), which describes how she sings.

This lesson is about the provisional object het, so why isn't het used here?

The provisional het is only used to anticipate a direct object that comes later in the sentence as a whole clause. In this sentence, the verb zingt is used without any direct object at all because she is just singing generally. If you added het here (Zij zingt het altijd heel zuiver), it would become a normal direct object meaning 'She always sings it perfectly in tune', referring to a specific song.