Learn Polish: Dopóki płacisz czynsz terminowo, możesz tu mieszkać. - As long as you pay rent on time, you can live here.

Dopóki translates to 'as long as' when the verbs in both parts of the sentence describe ongoing, simultaneous situations. Here, both płacisz (you pay) and możesz (you can) are imperfective present-tense verbs. If we wanted to say 'until', we would usually add the word nie and use a perfective verb, for example: dopóki nie zapłacisz (until you pay).

Why doesn't the word czynsz change its ending if it is the direct object of the verb płacisz?

It actually is in the accusative case here! Because czynsz (rent) is a masculine inanimate noun, its accusative form looks exactly the same as its nominative (dictionary) form. It only changes endings in other cases, like the genitive.

I usually say na czas for 'on time'. Why does this sentence use terminowo?

While both mean 'on time', terminowo specifically relates to meeting a set deadline or formal schedule (it comes from termin, meaning deadline or due date). It is the most natural word to use when talking about paying bills, rent, or completing contracts. Na czas is used for more general situations, like arriving at a train station on time.

Could I put tu at the very end of the sentence, like możesz mieszkać tu?