Learn French: Les vacances commencent bientôt.

In French, the word for a school or work break, les vacances, is almost always plural. Because it is a plural noun, it takes the plural article les (the).

Why does the verb commencent have an "-ent" at the end?

Since les vacances is plural, you have to treat it like "they". Commencent is the "they" form of the verb commencer (to start). Even though it has an -ent added to the end, those letters are completely silent, so it sounds exactly like the singular form commence.

How do I turn this sentence into a question asking when the vacation starts, using this lesson's rules?

To ask a "when" question, put the question word quand (when) at the very beginning, add est-ce que, and then plug in the rest of the statement. This gives you: Quand est-ce que les vacances commencent ?

Does bientôt literally mean "well early"?

It originally comes from joining bien (well/very) and tôt (early), but today it is just a standard, single word that simply means "soon".