Mater puerum monet ut nummos in crumena bene servet.

Because puerum is the direct object of monet. The mother is advising or warning the boy, so Latin puts puer into the accusative: puerum.

With moneo, the person who is warned/advised is often in the accusative.

After verbs of advising, urging, warning, ordering, and similar ideas, Latin often uses ut + subjunctive to express what someone is told to do.

So monet ut nummos in crumena bene servet means something like advises/warns him to keep the coins safely in the purse.

English often uses an infinitive, as in tells him to keep. Latin commonly uses ut + a subjunctive verb instead.

Because it is inside an ut clause depending on monet. In this construction, Latin uses the subjunctive.

So servet is not saying he keeps as a simple fact. It means that he should keep or to keep, which is why the subjunctive is used.

A learner will often first think of it as a purpose clause because it uses ut + subjunctive. But more precisely here it is usually called a substantive clause of indirect command.

That is because the clause gives the content of the warning/advice: what the mother tells the boy to do.

The understood subject of servet is the boy, the one referred to by puerum in the main clause.

Latin often leaves subject pronouns unstated because the verb ending already gives the person and number. Servet is third person singular, and the context makes it clear that the boy is the one who is supposed to keep the coins safe.

Because nummos is the direct object of servet. They are the things being kept safe.

So the sentence has:

• puerum = object of monet
• nummos = object of servet

Because in with the ablative usually means in/on in the sense of location, while in with the accusative usually means motion into.

Here the idea is keep the coins in the purse, meaning their location, so Latin uses the ablative: in crumena.

If the sentence meant something like put the coins into the purse, then in + accusative would be more likely.

Bene is an adverb, and it modifies servet. It tells us how the boy should keep the coins: well, carefully, or safely depending on context.

In Latin, adverbs can move around fairly freely, so bene does not have to stand in only one exact position.

No. Latin word order is much freer than English word order because the endings show the grammatical roles.

This sentence puts Mater first, which gives the subject a prominent place, and ends the subordinate clause with servet, which is very natural in Latin. But other orders are possible, such as:

Mater monet puerum ut bene nummos in crumena servet.

The meaning stays basically the same because the cases and verb forms make the relationships clear.

Here are the dictionary forms:

• mater = mother
• puer = boy
• moneo, monere, monui, monitum = warn, advise, remind
• nummus = coin
• crumena = purse, money-bag
• bene = well
• servo, servare, servavi, servatum = keep, preserve, guard

And the forms in the sentence are:

• Mater = nominative singular
• puerum = accusative singular
• monet = third person singular present active indicative
• nummos = accusative plural
• in crumena = prepositional phrase with ablative
• servet = third person singular present active subjunctive

It can mean several related things: warns, advises, reminds, or urges, depending on context.

So in this sentence, the exact English choice depends on the meaning already given to the learner. Grammatically, all of those fit the Latin structure well.