"The students sleep."
Hi! I am also learning it now but i think a little baground in German will help u understand this better . Depending on whether the noun is subject of the verb or object of the verb ,it undergoes transformation. Initial nouns are usually subject of the verb whilst after the verb they are usually objects.
discipulos is an accusative form. It is used in most cases as a direct object, the 'receiver' of the verb.
discipulos habent -> 'They have students.' (The students are being 'had').
discipuli here is nominative and is the subject of the sentence, the ones doing the verb.
discipuli habent -> 'The students have (something, I don't know, just an example, but the students are doing the having)'
The difference is similar to in English with pronouns. Example: they/them. 'They have students.' vs 'Students have them.'
I have responded to similar questions is this discussion, maybe those explanations with help as well.
Discipuli is used when the students are doing the action, as they are here 'doing the sleeping'.
Discipuli magistrum habent -> 'The students have a teacher' -> The students are the ones doing the action of having.
Discipulos is used when the action is being done to them.
Magister discipulos habet -> 'The teacher has students' -> The students are not the ones having, but rather the ones being had.
This is similar to in English with how some of the pronouns work.
'I see the teacher' compared to 'The teacher sees me'
'He loves her' compared to 'she loves him'
'They have a teacher' compared to 'The teacher has them'
Discipuli here is the nominative plural case. It is used as the subject of the sentence, the ones 'doing'/'performing' the verb action or being described.
Discipuli docent -> 'The students teach'.
Discipulos is the accusative plural case. It is used as the direct object of the sentence, often the thing that the verb action is being done to.
Discipulos docent -> "They teach the students"