Discipuli, qui iam sciunt quid sit subiectum et quid obiectum, facilius intellegunt cur “amari” passivum, “amare” activum sit.

The backbone of the sentence is:

• Discipuli ... intellegunt = The students ... understand

Everything between Discipuli and intellegunt adds extra information:

• qui iam sciunt... = who already know...

Then what they understand is given by an indirect question:

• cur amari passivum, amare activum sit = why amari is passive and amare is active

So the structure is:

• main clause: Discipuli facilius intellegunt
• relative clause describing discipuli: qui iam sciunt...
• indirect questions inside that relative clause and after intellegunt

Qui is a relative pronoun meaning who.

It refers back to discipuli:

• Discipuli, qui iam sciunt...
• The students, who already know...

Because discipuli is:

• masculine
• plural
• the subject of sciunt

the relative pronoun must also be nominative masculine plural: qui.

If it were feminine plural, it would be quae.
If it were object rather than subject, you might see quos.

Here quid means what in an indirect question:

• sciunt quid sit subiectum
• they know what a subject is

This is not the relative pronoun quod meaning which or that.
Instead, it is the interrogative pronoun quid, used after verbs like know, ask, understand, and so on.

So:

• quid sit subiectum = what a subject is
• quid obiectum = what an object is

English does the same thing:

• They know what a subject is

Because quid sit subiectum is an indirect question, and in Latin indirect questions normally use the subjunctive.

So Latin says:

• quid sit = what it is

not:

• quid est

This is very normal after verbs like:

• scio = I know
• nescio = I do not know
• quaero = I ask
• intellego = I understand

So:

• sciunt quid sit subiectum = they know what a subject is

The same thing happens later with cur ... sit.

Latin often leaves out a word when it is clearly understood from the previous part.

So:

• quid sit subiectum et quid obiectum

really means:

• quid sit subiectum et quid sit obiectum

The second sit is omitted because it would be obvious. English can do this too:

• They know what the subject is and what the object is
• or more briefly, They know what the subject is and what the object

Latin uses this kind of ellipsis very naturally.

For the same reason: cur introduces an indirect question.

• intellegunt cur ... sit
• they understand why ... is

In Latin, indirect questions usually take the subjunctive, so sit appears again.

Compare:

• direct question: Cur passivum est? = Why is it passive?
• indirect question: Intellego cur passivum sit. = I understand why it is passive.

That single sit applies to both parts. Latin is effectively saying:

• cur amari passivum sit, amare activum sit

But the first sit is omitted because it is easy to supply mentally.

So the sense is:

• why amari is passive and amare is active

This is another example of ellipsis.

Because the sentence is talking about the verb forms themselves.

• amare = to love, present active infinitive
• amari = to be loved, present passive infinitive

The point is that students who already know subject and object can more easily understand why:

• amari is passive
• amare is active

So the sentence is discussing grammar terminology and verb forms, not using those infinitives as the main verb of the sentence.

These are standard present infinitive forms of the 1st conjugation:

• active infinitive: -are
• passive infinitive: -ari

So from amo, amare:

• amare = to love → active
• amari = to be loved → passive

This pattern is very common in the 1st conjugation:

• laudare = to praise
• laudari = to be praised

They are being used as predicate words referring to the forms amari and amare.

Latin often treats a quoted word or infinitive as a neuter singular idea, something like:

• amari is a passive form
• amare is an active form

So passivum and activum are neuter singular to match that kind of grammatical label.

You can think of an understood noun like:

• verbum = word
• nomen = term
• infinitivum = infinitive form

So the sense is something like:

• amari passivum sit = amari is passive
• amare activum sit = amare is active

Facilius is the comparative adverb meaning more easily.

It modifies intellegunt:

• facilius intellegunt = they understand more easily

It comes from facile = easily.

So the sentence is not saying the students are easier; it is saying they understand more easily.

Iam means already.

So:

• qui iam sciunt
• who already know

It suggests that this knowledge comes first: once the students already know what subject and object are, they can more easily understand the difference between active and passive.

The relative clause is giving extra, descriptive information about the students:

• Discipuli, qui iam sciunt...
• The students, who already know...

This is similar to a non-restrictive relative clause in English.

The commas show that the sentence is talking about a particular set of students and adding information about them, rather than sharply distinguishing them from other students.

They are nominative singular, because they are the predicate nouns after sit.

In:

• quid sit subiectum

the idea is:

• what the subject is
• literally, what is a subject

Likewise:

• quid obiectum = what an object is

with sit understood.

So both words are standing as the thing being identified, not as direct objects of another verb.