Learn Polish: Zawsze trzeba szybko oddać dług. - One always has to pay back a debt quickly.

In this lesson, you are learning how od- indicates reciprocating an action or doing it back. Just as dzwonić is to call and oddzwonić is to call back, adding od- to dać (to give) creates oddać, meaning to give back, to return, or in this context, to pay back.

The word zawsze usually triggers an imperfective verb. Why do we use the perfective oddać here instead of oddawać?

This is a tricky exception! While zawsze (always) signals a repeated situation, the rule of using imperfective for repetition sometimes gets overridden by the need to stress the successful completion of the action. Here, the phrase means that whenever you have a debt, you must completely pay it off. The focus is on the finished result of giving it back each time, hence the perfective oddać.

What case is the word dług, and why hasn't its ending changed?

It is in the accusative case because it is the direct object of the verb oddać (it is the thing being paid back). Because dług (debt) is a masculine inanimate noun, its accusative form is exactly the same as its nominative, or dictionary, form.

Could I change the word order and say Zawsze trzeba oddać dług szybko?