Post ientaculum puella in lecto iacet et librum legit, quia ei pectus dolet.

It is in the correct case already. Post takes the accusative case, and ientaculum is a neuter second-declension noun. In the neuter singular, the nominative and accusative forms are the same.

So:

• ientaculum = nominative singular breakfast
• ientaculum = accusative singular breakfast

After post, it is accusative: post ientaculum = after breakfast.

Because puella is the subject of the main verbs iacet and legit.

In other words, the girl is the one who:

• lies in bed
• reads a book

So Latin uses the nominative case: puella.

Because Latin uses in with two different cases depending on the meaning:

• in + ablative = in/on a place, showing location
• in + accusative = into/onto a place, showing motion toward

Here the girl is already in bed, not moving into it, so Latin uses the ablative:

• in lecto = in bed

If it were in lectum, it would mean into bed.

Iacet means lies or is lying.

It comes from iacere, meaning to lie, to be lying down, or to be stretched out. In this sentence it suggests that the girl is resting or lying in bed.

It is:

• present tense
• 3rd person singular
• active indicative

So: she lies / she is lying.

Because librum is the direct object of legit.

The verb legere means to read, and the thing being read goes into the accusative case.

So:

• liber = book (nominative)
• librum = book (accusative, as the object)

Thus librum legit = she reads a book.

Good question: legit can be ambiguous by itself.

It can be:

• present: he/she reads
• perfect: he/she read

But in this sentence, the context makes present tense the natural choice, because it appears alongside other present-tense verbs:

• iacet = is lying
• legit = reads / is reading
• dolet = hurts / is hurting

So here legit means reads or is reading.

Because Latin often leaves the subject understood when it stays the same.

Here, once puella has been named, the next verb legit naturally continues with the same subject unless something signals a change.

So Latin does not need to say:

• puella iacet et puella librum legit

It can simply say:

• puella ... iacet et librum legit

This is very normal Latin style.

Because Latin word order is much more flexible than English word order.

English relies heavily on word order to show who is doing what. Latin relies much more on case endings and verb endings. That means Latin can move words around for emphasis, rhythm, or style.

In this sentence:

• Post ientaculum comes first to set the time
• puella identifies the subject
• in lecto gives the location
• the verbs come later

A more English-like order would be possible in theory, but the given order is very natural Latin.

Quia means because. It introduces a subordinate clause of reason.

So everything after quia explains why the girl is lying in bed reading:

• quia ei pectus dolet = because her chest hurts

It works much like English because.

Because ei is dative, while eam is accusative, and the verb dolet does not treat the person in pain as a direct object here.

In Latin, with dolere, a common pattern is:

• the body part is the subject
• the person affected is in the dative

So:

• ei pectus dolet

literally means something like:

• to her, the chest hurts

Natural English turns that into:

• her chest hurts

So ei is correct because the girl is the person affected, not the direct object.

Because Latin often expresses this idea through the dative of the person affected, rather than by adding a possessive adjective.

So instead of saying something exactly like her chest hurts, Latin often says:

• to her, the chest hurts

That is what ei pectus dolet does.

This is a very common Latin way to talk about pain or bodily discomfort.

Because sibi is a reflexive pronoun, and reflexive pronouns normally refer back to the subject of their own clause.

In the clause:

• quia ei pectus dolet

the grammatical subject is pectus, not puella.

So Latin does not use the reflexive sibi here. It uses the ordinary pronoun ei to refer to the girl.

This can feel strange to English speakers, because English does not usually think about pronouns this way, but it is standard Latin grammar.

Pectus is the subject because it is the thing that hurts.

So Latin structures the idea as:

• pectus dolet = the chest hurts

Since pectus is singular, the verb must also be singular:

• dolet = hurts

Also, pectus is a useful noun to notice because it is neuter third declension. Even though it ends in -us, it is not a regular masculine second-declension noun like servus.

Its dictionary form is:

• pectus, pectoris = chest, breast

Because classical Latin has no articles.

So Latin does not have separate words corresponding exactly to English:

• the
• a/an

Whether puella means the girl or a girl, and whether librum means the book or a book, depends on the context.

That is completely normal in Latin.