Post lectionem magistra puellas ad ripam ducit, ubi parva navicula prope pontem ligata est.

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

So:

• lectio = lesson (dictionary form)
• lectionem = lesson in the accusative singular

That is why post lectionem means after the lesson.

This is something Latin learners often have to memorize: many prepositions regularly take a specific case, and post takes the accusative.

The endings tell you.

• magistra is nominative singular, so it is the subject: the teacher
• puellas is accusative plural, so it is the direct object: the girls

So magistra puellas ducit means the teacher leads the girls.

Even if the word order changed, the case endings would still show who is doing the action and who receives it.

Because ad is the preposition meaning to or toward, and it is commonly used to show motion toward a place.

• ad
  • accusative = to/toward
• ripam is the accusative singular of ripa = bank, shore

So:

• ad ripam = to the bank

Latin often uses a preposition where English also does, but the important thing is that ad requires the accusative.

Because prope can function as a preposition meaning near, and when it does, it takes the accusative.

So:

• pons = bridge
• pontem = accusative singular
• prope pontem = near the bridge

This sometimes surprises English speakers because near in English does not affect the form of the noun, but in Latin the preposition determines the case.

Here ubi means where.

It introduces a clause describing the place the teacher leads the girls to:

• ad ripam ducit = she leads them to the bank
• ubi ... ligata est = where ... is tied

So the second clause tells you what is at that bank: a small boat is tied near the bridge there.

Depending on context, ubi can also mean when, but here where is the natural meaning.

Ligata est is made of:

• ligata = the perfect passive participle of ligare (to tie)
• est = is

Together they form the perfect passive of the verb:

• ligata est = literally has been tied

But in many contexts, especially with something physically fastened or positioned, English often translates it more naturally as:

• is tied
• is moored
• has been tied up

So grammatically it is a passive form, and idiomatically it often describes the resulting state.

Because ligata must agree with navicula.

• navicula is feminine singular
• so the participle describing it must also be feminine singular
• therefore: ligata

If the noun were masculine singular, you would expect ligatus.
If it were neuter singular, you would expect ligatum.

This is normal adjective/participle agreement in Latin: the describing word matches the noun in gender, number, and case.

Navicula means small boat or little boat.

It is a diminutive form related to navis (ship, boat). The ending gives the sense of something smaller or more modest.

So:

• navis = ship/boat
• navicula = small boat, little boat

The adjective parva also means small, so parva navicula is something like a small little boat or more naturally a small boat. Latin can use both the diminutive noun and an adjective together for vividness.

Because it is the subject of the second clause.

In ubi parva navicula prope pontem ligata est, the thing being described as tied is the small boat. So it is the subject, and subjects are normally in the nominative.

• parva = nominative feminine singular
• navicula = nominative feminine singular

Together they mean a small boat.

Latin word order is much more flexible than English word order because case endings show grammatical function.

Placing the verb at or near the end of the clause is very common in Latin prose:

• Post lectionem magistra puellas ad ripam ducit

A very literal order would be:

• After the lesson the teacher the girls to the bank leads

That sounds strange in English, but it is perfectly normal in Latin.

The ending of magistra shows it is the subject, and the ending of puellas shows it is the object, so Latin does not need a fixed subject-verb-object order the way English usually does.

For the same general reason: Latin often places the verb late in the clause, sometimes at the very end.

• ubi parva navicula prope pontem ligata est

This arrangement can feel especially natural in Latin when the sentence builds up to the verb. English usually prefers the verb earlier, but Latin often saves it for last.

So both ducit and est are in very typical Latin positions.

Yes. Post lectionem tells you when the action happens: after the lesson.

It is an adverbial phrase of time modifying the main verb ducit.

So the sentence structure is roughly:

• When? after the lesson
• Who? the teacher
• Whom? the girls
• Where to? to the bank
• What is there? a small boat tied near the bridge

This is a good example of how Latin often packs several kinds of information into one sentence through case endings and short phrases.