Post longam horam discipula lassa dormit.

Because post takes the accusative case in Latin when it means after.

• post = after
• horam = accusative singular of hora
• longam = accusative singular feminine of longus, agreeing with horam

So post longam horam means after a long hour.

A useful comparison:

• longa hora = nominative, a long hour
• longam horam = accusative, used here because of post

Because it describes discipula, which is a feminine singular noun.

Latin adjectives must agree with the nouns they describe in:

• gender
• number
• case

So:

• discipula = feminine singular nominative
• lassa = feminine singular nominative

They match, so lassa means tired describing the female student / schoolgirl.

If the subject were masculine, you would expect:

• discipulus lassus dormit = the tired male student sleeps

Because discipula is the subject of the sentence, and the subject of a finite verb is normally in the nominative case.

Here:

• discipula = the student
• dormit = sleeps

So discipula is the one doing the action of sleeping.

That is why it is nominative, not accusative.

Because the subject, discipula, is singular.

• discipula = one female student
• dormit = she sleeps / the student sleeps

Latin verbs change form to match the subject:

• dormit = he/she/it sleeps
• dormiunt = they sleep

So if the subject were plural, you would get something like:

• discipulae lassae dormiunt = the tired female students sleep

Primarily, it is an adjective describing discipula: the tired student.

So the phrase is naturally understood as:

• discipula lassa = the tired student

In context, Latin adjectives like this can also feel slightly descriptive in a looser way, almost like:

• the student, tired, sleeps

But for a learner, the safest understanding is that lassa simply modifies discipula.

Because Classical Latin has no articles.

So discipula can mean:

• a student
• the student

and longam horam can mean:

• a long hour
• the long hour

Which one sounds best depends on context and translation.

English requires an article, but Latin usually does not.

Yes. Latin word order is much freer than English word order because the endings show each word’s role.

So all of these could express basically the same idea:

• Post longam horam discipula lassa dormit.
• Discipula lassa post longam horam dormit.
• Dormit post longam horam discipula lassa.

The exact emphasis may change, but the grammar stays clear because of the endings:

• discipula = nominative subject
• longam horam = accusative after post
• dormit = verb

Here it means after a long hour, because of post.

That is an important distinction:

• post longam horam dormit = she sleeps after a long hour
• longam horam dormit = she sleeps for a long hour

So without post, the accusative can express duration of time.
With post, it means after.

Because it agrees with horam, and hora is feminine.

So:

• hora = feminine noun
• horam = accusative singular feminine
• longam = accusative singular feminine adjective

The adjective must match the noun it describes.

That is why Latin uses longam horam, not longum horam or longus horam.

No. Discipula is specifically feminine.

• discipula = female student
• discipulus = male student

So this sentence refers to a female student unless the exercise is using discipula in a very broad school-context translation like student, while still keeping the grammatical femininity.

Because of both meaning and grammar.

Grammatically:

• discipula is nominative singular feminine
• lassa is nominative singular feminine

So they match.

But hora in this sentence is not nominative; it is horam, accusative singular feminine because it follows post. If lassa described horam, it would also need to be accusative:

• post longam lassam horam = after a long tiring hour or after a long tired hour depending on context, though that would be a different phrase

Since we have lassa, not lassam, it clearly goes with discipula.

Dormit is:

• third person singular
• present tense
• active voice
• indicative mood

So it means:

• he sleeps
• she sleeps
• it sleeps

In this sentence, because the subject is discipula, it means she sleeps or the student sleeps.

Because the verb ending already tells you the person and number.

• dormit already means he/she/it sleeps

Latin often leaves subject pronouns out unless they are needed for emphasis or contrast.

So:

• dormit = she sleeps
• discipula dormit = the student sleeps
• ea dormit = she sleeps, with the pronoun stated for emphasis or clarity

That is why a separate word for she is unnecessary here.