Learn Croatian: Moram pregledati sav posao. - I must inspect all the work.

Sav is the masculine form of the word for "all." Because posao (work/job) is a masculine noun, the describing word must match its gender. If the noun were feminine, you would use sva, and if it were neuter, you would use sve.

Is sav posao in the accusative case here?

Yes, it is the direct object receiving the action of the verb pregledati (to inspect). For masculine non-living nouns like posao, the accusative case looks exactly the same as the nominative (dictionary) form. Because of this, both the noun and sav keep their basic forms instead of taking an ending.

Why is the second verb, pregledati, in its infinitive form?

It follows the conjugated verb moram (I must). Whenever you use a modal verb like morati (to have to / must), the verb that comes right after it stays in the infinitive, which is the dictionary form that usually ends in -ti or -ći.

Can I use cijeli (whole) instead of sav in this sentence?

Yes, they are very similar here! You could say Moram pregledati cijeli posao (I must inspect the whole job). Sav emphasizes looking at "all" the work, while cijeli emphasizes the "entirety" of the job, but native speakers use both naturally in this context.