Learn French: Une partie des étudiants est absente. - Some of the students are absent.

In French, the grammatical subject of this sentence is Une partie (a part), which is singular. Even though it refers to multiple students, the verb must match this singular noun, so we use est instead of the plural sont.

Why does absente have an extra "e" at the end if étudiants is masculine?

Just like the verb, the adjective absente must agree with the core subject of the sentence, which is Une partie. Since partie is a feminine singular noun, the adjective takes the feminine singular ending -e.

Where does the word des come from in this sentence?

It is a mandatory contraction. The phrase translates literally to "a part of the students". In French, "of" is de and the plural "the" is les. Whenever de and les are next to each other, they always combine to form des.

How would I change this sentence to say "All the students are absent" using the grammar from this lesson?

You would use the plural indefinite adjective Tous followed by the definite article les. The sentence becomes Tous les étudiants sont absents. Notice that the verb and adjective now become plural because the subject is the students themselves!