Cum remex ratem lente ageret, portitor puellae ostendit ubi alveus altior esset.

Here cum introduces a subordinate clause that gives the background for the main action. In this kind of sentence, it is often translated when or while.

So Cum remex ratem lente ageret means something like while/when the rower was moving the boat slowly.

Because this is a cum clause in past narration. When cum gives the circumstances or background of a past event, Latin commonly uses the subjunctive.

This is often called a circumstantial cum clause or narrative cum clause.

So:

• cum ... ageret = while/when ... was steering/moving

The imperfect shows an action that was ongoing in the past. It gives the background scene rather than a single completed action.

So ageret suggests:

• was driving / was steering / was moving

rather than:

• drove
• steered

It matches the idea that the rower was in the middle of doing this when the ferryman showed the girl something.

Agere has a very broad range of meanings, including do, drive, lead, and set in motion. With something like a boat, it can mean to drive, to guide, or to steer.

So ratem agere means to drive/guide the boat.

Ratem is the accusative singular of ratis, a third-declension noun meaning raft or boat.

It is accusative because it is the direct object of ageret:

• the rower was steering the boat

So:

• nominative: ratis
• accusative: ratem

Puellae is dative singular.

It is the indirect object of ostendit:

• portitor puellae ostendit = the ferryman showed the girl / showed to the girl

English often does not visibly mark this difference, but Latin does.

Ubi means where here, and it introduces an indirect question after ostendit.

So the structure is:

• ostendit ubi ... esset = he showed where ... was

This is not a simple relative clause. It is a real question embedded inside the sentence:

• Where was the channel deeper?
• He showed her where it was.

Because ubi alveus altior esset is an indirect question, and Latin regularly uses the subjunctive in indirect questions.

So esset is subjunctive not because of ubi alone, but because the whole clause depends on ostendit:

• he showed [her] where the channel was deeper

Since the main verb ostendit is past, Latin normally uses the imperfect subjunctive in an indirect question for action or situation that is simultaneous with that past main verb.

So:

• ostendit ubi alveus altior esset
• he showed where the channel was deeper

This is standard sequence of tenses.

Altior is the comparative of altus. Depending on context, altus can mean either high or deep.

With a river, channel, or water depth, English naturally translates it as deeper.

So:

• alveus altior = literally a higher/deeper channel
• in natural English: a deeper channel

Because alveus is nominative singular, which is the normal case for the subject.

Also, altior agrees with it:

• alveus = nominative singular masculine
• altior = nominative singular masculine comparative adjective

So the phrase means:

• the channel was deeper

By contrast, puellae is dative, so it cannot be the subject of esset.

To an English speaker, yes, but it is perfectly normal Latin.

Latin word order is much freer because endings show each word’s grammatical role. A very literal English arrangement would be:

• Cum remex ratem lente ageret
  While the rower the boat slowly was steering
• portitor puellae ostendit ubi alveus altior esset
  the ferryman to the girl showed where the channel deeper was

Latin does not need to keep a rigid English-style order like subject–verb–object.

Without macrons, ostendit can look the same in writing whether it is:

• present: shows
• perfect: showed

The context tells you which one it is. Here the sentence is clearly in past narration:

• ageret is imperfect subjunctive
• esset is imperfect subjunctive
• the whole sentence describes a past scene

So ostendit is best understood as showed.

They are two different people with boat-related jobs.

• remex = rower, someone who rows
• portitor = ferryman or boatman, someone who carries people across

So the sentence distinguishes between:

• the person rowing or handling the boat at that moment
• the ferryman, who shows the girl where the channel is deeper

Depending on context, they could be different roles in the same scene.