Learn Croatian: Zašto uvijek odgađaš posao i gubiš vrijeme? - Why do you always postpone work and waste time?

This sentence doesn't use umjesto da (instead of). How could I rewrite it to say 'Why do you waste time instead of doing the work?' using the current lesson's grammar?

You would use umjesto da followed by a present tense verb. For example: Zašto gubiš vrijeme umjesto da radiš posao? (Why do you waste time instead of doing the work?).

Why is odgađaš used here instead of odgodiš?

Odgađaš is the imperfective form of the verb, used for ongoing or repeated actions. Because the sentence includes uvijek (always), it describes a habitual action, making the imperfective aspect necessary here.

Does gubiš vrijeme literally mean 'losing time'?

Yes, the verb gubiti literally translates to 'to lose'. In Croatian, gubiti vrijeme is the standard, everyday expression for 'to waste time'.

What case are posao and vrijeme in here?

They are both in the accusative case because they are the direct objects of the verbs. Because posao is an inanimate masculine noun and vrijeme is a neuter noun, their accusative forms are identical to their nominative (dictionary) forms.