Post brevem requiem puer citius scribit.

Because post is a preposition that takes the accusative case when it means after in a temporal sense.

So in this sentence:

• post = after
• brevem requiem = a short rest

Both brevem and requiem are in the accusative because they belong to the phrase governed by post.

Because brevem has to agree with requiem.

The adjective is brevis, breve = short.
Its dictionary form is usually given in the nominative, but here it must match the noun it describes:

• requies = rest, repose
• accusative singular = requiem

Since requiem is:

• feminine
• singular
• accusative

the adjective must also be:

• feminine
• singular
• accusative

So:

• nominative: brevis requies
• accusative: brevem requiem

Because requies is the nominative singular form, but after post Latin needs the accusative singular.

This noun belongs to the fifth declension:

• nominative singular: requies
• accusative singular: requiem

So post requiem means after a rest.

English does not change the form of rest, but Latin does.

You know it from the nominative case and from the verb ending.

• puer = boy
• It is nominative singular, so it can be the subject.
• scribit means he writes or the boy writes

These fit together naturally:

• puer scribit = the boy writes

Even though English relies heavily on word order, Latin often tells you the job of a word by its ending instead.

Scribit is:

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

It comes from scribo, scribere = to write.

So scribit means:

• he writes
• she writes
• it writes

In this sentence, because the subject is puer, it means the boy writes.

Yes. Citius is an adverb, specifically the comparative form.

It comes from the adverb cito = quickly.
Its comparative is citius = more quickly, faster.

So:

• cito = quickly
• citius = more quickly / faster

Latin often uses a comparative adverb even when the second half of the comparison is not stated explicitly. English can do this too:

• After a short rest, the boy writes faster.

This may imply faster than before or faster than someone else, depending on context.

Classical Latin does not have articles like English the and a/an.

So a noun like puer can mean:

• the boy
• a boy

and requiem can mean:

• a rest
• the rest

The context or the provided translation tells you which English article is best.

The basic meaning stays the same even if the word order changes, because Latin endings show the grammatical roles.

For example, these would mean essentially the same thing:

• Post brevem requiem puer citius scribit.
• Puer post brevem requiem citius scribit.
• Citius puer post brevem requiem scribit.

However, word order can change emphasis.

In your sentence, beginning with post brevem requiem highlights the time setting first: after a short rest...

That is very natural in Latin.

Yes. It is a prepositional phrase expressing time.

The core sentence is:

• puer scribit = the boy writes

Then Latin adds:

• post brevem requiem = after a short rest
• citius = more quickly

So the structure is roughly:

• time phrase
  • subject
    • adverb
      • verb

Because the verb ending already includes the person and number.

In scribit, the ending -it tells you it is third person singular. So Latin does not need a separate pronoun like English he.

That means:

• scribit by itself can mean he writes, she writes, or it writes
• puer scribit makes the subject explicit: the boy writes

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

A common classroom pronunciation would be roughly:

post BREH-wem REK-wee-em POO-er KEE-tee-oos SKREE-bit

A few helpful points:

• v is usually pronounced like w in restored classical pronunciation
• qu is pronounced like kw
• c is always hard, like k
• ti in citius is normally pronounced as written, not like English sh

So:

• brevem ≈ BREH-wem
• requiem ≈ REK-wee-em
• puer ≈ POO-er
• citius ≈ KEE-tee-oos
• scribit ≈ SKREE-bit

Yes. Adverbs are fairly flexible in Latin.

You can have:

• puer citius scribit
• puer scribit citius

Both mean essentially the same thing. Placing citius before the verb is very common and often feels natural, because it closely modifies the action of writing.

So in this sentence, puer citius scribit means the boy writes more quickly with no special difficulty.