Learn Dutch: U bent ook moe, meneer. - You are also tired, sir.

u is the formal word for "you", and it can actually be used for both singular (one person) and plural (multiple people)! Because this sentence addresses one meneer (sir), it is singular here. Notice that it uses bent, just like the informal jij bent (you are).

Can I use jullie instead of u since this lesson is about plural pronouns?

You wouldn't use jullie (plural "you") because you are only talking to one person (meneer). If you were talking to a group of men informally, you could say Jullie zijn ook moe (You are also tired). However, if you wanted to stay formal with a group, you would still just use u!

Why does ook (also) come before moe (tired)?

In Dutch, short extra words like ook (also) or niet (not) usually come right after the verb and before the adjective they describe. So it flows just like English here: U bent ook moe ("You are also tired").

I sometimes see u is instead of u bent. Are both correct?

Yes! For the formal "you" (u), both u bent and u is mean "you are" and both are grammatically correct. However, u bent is much more common in everyday spoken Dutch.