Discipula de epistula matris diu cogitat.

Because discipula is in the nominative singular, the case normally used for the subject of a sentence.

Also, the verb cogitat means he/she/it thinks / is thinking, so it matches a singular subject. Since discipula means female student, it is the person doing the thinking.

Because the preposition de takes the ablative case.

So:

• epistula = nominative singular
• epistulam = accusative singular
• epistulā = ablative singular

In normal spelling, the long vowel is often not marked, so you may simply see epistula, but here it is functioning as ablative singular after de.

So de epistula means about the letter.

Matris is the genitive singular of mater, meaning of the mother.

Latin often shows possession or close relationship by using the genitive:

• epistula matris = the letter of the mother
• more naturally in English: the mother’s letter or the letter from the mother

So matris depends on epistula.

It can mean either, depending on context.

Latin epistula matris literally means letter of the mother, but in natural English that may be expressed as:

• the mother’s letter
• a letter from the mother
• the letter from her mother

Latin itself does not force one single English phrasing here.

Diu is an adverb meaning for a long time or for a long while.

It modifies the verb cogitat, so it tells you how long the student is thinking.

So the sense is not just she thinks about the letter, but she thinks about the letter for a long time.

Because the Latin present tense can cover both ideas that English often separates:

• she thinks
• she is thinking

So cogitat can be translated either way, depending on context and style.

In this sentence, because of diu (for a long time), English may naturally prefer is thinking or has been thinking, but the Latin form itself is simply present tense.

Latin word order is more flexible than English word order because the case endings show how the words function.

So this sentence could also appear as:

• Discipula diu de epistula matris cogitat
• De epistula matris discipula diu cogitat
• Diu discipula de epistula matris cogitat

All of these are grammatically possible. The basic meaning stays the same, though emphasis can shift a little.

The given order is perfectly natural.

For think about, Latin commonly uses cogitare de + ablative.

So:

• de epistula cogitat = she thinks about the letter

If you remove de, the sentence would no longer be the normal straightforward way to say think about in this context.

So learners should strongly associate:

cogitare de aliqua re = to think about something

Latin has no articles like English the or a/an.

That means discipula can mean:

• the student
• a student

and epistula matris can mean:

• the mother’s letter
• a letter from the mother

Context tells you which English article fits best.

A learner often wants to identify the base forms:

• discipula → discipula, discipulae = female student
• epistula → epistula, epistulae = letter
• mater / matris → mater, matris = mother
• diu = for a long time
• cogitat → cogito, cogitare, cogitavi, cogitatum = think

This is especially useful because matris may look unfamiliar until you recognize it as a form of mater.

Because they are both first-declension nouns, and in the singular the nominative and ablative forms can look very similar in ordinary spelling.

Here:

• discipula is nominative singular: the subject
• epistula is ablative singular after de

Traditionally, with vowel lengths marked, you would see:

• discipula = nominative
• epistulā = ablative

But many printed Latin texts do not mark long vowels, so you learn the function from the grammar of the sentence, not just the spelling.