Mater puerum monet ut cautus sit, cum viam transeat.

Because puerum is the direct object of monet.

The verb moneo means warn, advise, remind, and in this sentence the mother is warning the boy, so the boy is the person acted upon:

• Mater = the mother, subject
• puerum = the boy, direct object
• monet = warns/advises

So Latin uses the accusative: puerum.

Here monet means something like warns, advises, or cautions.

The verb moneo has a range of meanings in Latin, including:

• remind
• advise
• warn

In this sentence, because it is followed by ut cautus sit, the sense is not just reminds, but more specifically warns/advises him to be careful.

It is an indirect command clause, also called a substantive clause of command.

After verbs like warn, advise, order, urge, and similar ideas, Latin often uses:

ut + subjunctive

So:

• monet ut cautus sit = warns/advises him to be careful

English often uses an infinitive here, as in to be careful, but Latin normally prefers ut plus a subjunctive verb.

Because it is in an indirect command introduced by ut.

After monet, Latin does not usually say the equivalent of warns him is careful. Instead, it uses a clause expressing what the person is being told or urged to do:

• ut cautus sit = that he be careful / to be careful

The verb in that kind of clause is subjunctive, so we get sit, not est.

It is specifically the present subjunctive, which fits well with the present main verb monet.

Because cautus is a predicate adjective with sit, and it agrees with the understood subject of sit, not with puerum as a direct object.

The understood subject is the boy:

• ut cautus sit = that he be careful

So cautus is:

• masculine
• singular
• nominative

It matches the implied he = puer.

This is a very common point for English speakers, because in English we often think first about the object the boy, but in Latin the adjective after sum agrees with the subject of that clause.

It is understood rather than stated.

Latin often leaves out subject pronouns when the meaning is clear from the context and the verb ending. Here the boy is already mentioned as puerum, so Latin does not need to repeat he.

Both verbs are third-person singular:

• sit = he may be / he should be
• transeat = he may cross / he crosses

So the subject of both is understood to be the boy.

No. Here cum is a conjunction meaning when.

Latin cum can be two different words:

• cum as a preposition = with
• cum as a conjunction = when, sometimes since or although

In this sentence it is clearly the conjunction:

• cum viam transeat = when he crosses the road

So this is not the preposition with.

Because the cum clause is part of the situation being described in the warning, not just a simple stand-alone statement of fact.

In other words, the mother is warning him to be careful when he crosses the road. Latin commonly uses the subjunctive in this kind of dependent cum clause, especially when it gives the circumstance in which the action should happen.

So:

• cum viam transeat = when/whenever he crosses the road

The present subjunctive also fits the present-time framework set by monet.

Because viam is the direct object of transeat.

The verb transeo means cross or go across, and in Latin it can take a direct object:

• viam transire = to cross the road

So viam is accusative because it is the thing being crossed.

Because Latin word order is flexible, and the endings show the grammar more clearly than the position does.

This order is very natural:

• Mater — starts with the subject
• puerum monet — gives the main action
• ut cautus sit — gives what she warns him to do
• cum viam transeat — adds the circumstance

English relies much more heavily on word order, but Latin can move these parts around for emphasis without changing the basic meaning, as long as the endings stay the same.