Learn German: Seid ihr müde?

Seid is the specific form of the verb sein (to be) that matches ihr (you, plural). Just like in English you say "I am" but "you are", German changes the verb depending on who is doing the action. You use du bist for one person, and ihr seid for two or more people.

Wait, doesn't ihr mean "her" or "their"?

It can! The word ihr has a few different meanings depending on the context. Here, as the subject of the sentence paired with the verb seid, it means the informal plural "you" (like "you guys" or "y'all"). You use this when talking directly to multiple people you are on a first-name basis with.

Since ihr refers to multiple people, shouldn't the adjective müde have a plural ending?

No, and this makes things much easier! In German, adjectives only change their endings when they come directly before a noun. When an adjective is separated from the subject by a verb like sein (to be), it just stays in its basic dictionary form. It does not matter if the subject is singular or plural.

Why does the verb seid come at the very beginning of the sentence?