Learn German: Der Uhrmacher muss alles justieren, sei es das Zahnrad oder der Zeiger. - The watchmaker must adjust everything, whether it be the gear or the pointer.

It is a fixed phrase meaning whether it be ... or .... It uses sei, which is the Subjunctive I (Konjunktiv I) form of sein (to be). Even though Konjunktiv I is usually used for indirect speech, here it functions in a set expression to show alternative possibilities, also known as a concession.

Why are das Zahnrad and der Zeiger in the nominative case? Shouldn't they be accusative since they are the things being adjusted?

That is a great observation! Even though they logically specify alles, which is the direct object (accusative) of justieren, they are grammatically linked to the verb sein in the phrase sei es. Because the verb sein always takes the nominative case, we must use the nominative forms das Zahnrad and der Zeiger.

The lesson topic mentions sei es nun. Could I add nun to this sentence?

Yes, you can easily say sei es nun das Zahnrad oder der Zeiger. Adding the modal particle nun makes the phrase sound a bit smoother and more natural in spoken German, but it does not change the core meaning of the sentence at all.

Is justieren a common everyday word, or would you normally say something like einstellen?