Broadly, there are two types of verb: stative and active.

Stative verbs, also called copulae, are verbs of state. They describe the state of the subject.

The cake smells delicious.
She looks tired.
They sound angry.
He is tall.
That seems reasonable.
The woman became a teacher.

Usually, anything that comes after a stative verb is put in the subject form (a.k.a. the nominative). In languages with agreement, any adjectives in the predicate will agree with the subject in exactly the same way as they do when they're part of the same noun phrase:

la bona knabino
la bonaj knabinoj
la knabino estas bona
la knabinoj estas bonaj

Active verbs are verbs of action, and they come in two flavors: intransitive and transitive. Think of the word "transit". It suggests motion from one place to another.

Intransitive verbs are self-contained. There is nothing to receive their action.

She sleeps.
He runs.
They talk.
I jump.

Transitive verbs perform an action on a thing, and the thing that receives the action is the direct object. The case it takes is called the accusative.

He threw the ball. (What was thrown?)
She eats dinner. (What was eaten?)
I like him. (Who is liked?)
He sees her. (Who is seen?)
Let's ask them. (Who is asked?)

Only sentences with transitive verbs can be re-written into the passive voice, which is hinted at when put in to question form as above:

The ball was thrown.
Dinner was eaten.
He is liked.
She is seen.
They are asked.

Ŝi kisas lin.
Li kisas ŝin.
Mi trinkas lakton.
Ni manĝas ĉokoladon.