Bonum discipulum magistris parere decet.

Because decet commonly takes the person concerned in the accusative.

So:

• bonum discipulum = a good student in the accusative
• decet = it is fitting / it is proper

The idea is not quite the good student does something, but rather it is proper for a good student to do something.

So the structure is:

• bonum discipulum ... decet = it befits a good student ...

Because the adjective has to agree with the noun in gender, number, and case.

Here:

• discipulum is masculine singular accusative
• so bonum must also be masculine singular accusative

That is why you get:

• bonus discipulus = a good student as a nominative subject
• bonum discipulum = a good student in the accusative

Because after decet, Latin often uses an infinitive to say what it is proper to do.

So:

• parere = to obey
• bonum discipulum magistris parere decet = it is proper for a good student to obey teachers

The infinitive phrase magistris parere functions like to obey teachers in English.

Because pareo, parere takes the dative case, not the accusative.

So Latin says:

• magistris parere = literally to obey to the teachers or to be obedient to the teachers

But in normal English we simply say:

• to obey the teachers

This is very common in Latin: some verbs that take a direct object in English instead take the dative in Latin.

Decet comes from decere, meaning:

• to be fitting
• to be proper
• to be suitable
• to be becoming

So bonum discipulum magistris parere decet can be understood as:

• It is proper for a good student to obey teachers
• It befits a good student to obey teachers

Befits is closer to the Latin, though it is proper is more natural for many learners.

In English, we usually say:

• It is proper for a good student to obey teachers

Latin does not need a separate word for that it.

The verb decet by itself can express the idea. The infinitive phrase magistris parere supplies the content of what is fitting.

So English uses a dummy subject it, but Latin does not have to.

Because Latin has no articles.

So discipulum can mean, depending on context:

• a student
• the student
• sometimes just student

Here, English naturally translates the sense as:

• a good student

But Latin does not need a separate word for a.

Latin word order is much more flexible than English word order because the endings show the grammatical relationships.

This sentence is arranged as:

• Bonum discipulum = a good student
• magistris parere = to obey teachers
• decet = it is proper

A very literal English rendering would be:

• A good student to obey teachers is fitting

That sounds unnatural in English, so we translate more naturally:

• It is proper for a good student to obey teachers

Putting decet at the end is completely normal Latin style.

Yes. For example, Latin could also say:

• Bonum discipulum decet magistris parere
• Magistris parere bonum discipulum decet

The core meaning would stay the same, because the endings still show the roles:

• bonum discipulum = accusative
• magistris = dative
• parere = infinitive
• decet = verb

Different word orders mainly change emphasis or style, not the basic meaning.

The form magistris could, by itself, be either dative plural or ablative plural.

But in this sentence it must be dative, because parere takes the dative.

So here:

• magistris = to/for the teachers grammatically
• in idiomatic English: the teachers

This is a good example of how you often identify a case not just from the ending, but from the verb it goes with.