Puer malleum tenere vult, sed pater eum monet ne clavos male figat.

Because malleum is the direct object of tenere.

• tenere = to hold
• the thing being held is the hammer
• in Latin, a direct object usually goes in the accusative

So:

• malleus = hammer as a subject
• malleum = hammer as an object

In this sentence, the boy wants to hold the hammer, so malleum must be accusative.

Because Latin often uses an infinitive after verbs like vult.

• vult = he wants
• tenere = to hold

So tenere vult literally means wants to hold.

This is very common in Latin. It is similar to English:

• He wants to hold
• Boy wants to hold the hammer

The infinitive after a verb like vult is sometimes called a complementary infinitive, because it completes the meaning of the first verb.

Eum means him, and it refers back to puer.

So:

• puer = the boy
• eum = him = the boy

It is in the accusative because it is the direct object of monet:

• pater eum monet = the father warns him

Latin often uses a pronoun like this instead of repeating the noun.

Here monet means warns or advises.

The pattern is:

• aliquem monet = he warns someone
• often followed by ne
  • subjunctive = not to ...

So:

• pater eum monet = the father warns him
• ne clavos male figat = not to fasten/drive the nails badly

Put together:

• pater eum monet ne clavos male figat = the father warns him not to drive the nails badly

This is a very common Latin construction.

Because ne introduces a negative idea after a verb of warning.

Here it means something like:

• that he should not
• or more naturally in English, not to

So:

• monet ne figat = warns him not to fasten/drive

English uses not to, but Latin often uses ne with a subjunctive verb.

A learner may expect non, but non usually just negates a word or statement directly. After verbs like monet, Latin normally uses ne in this kind of clause.

Because after monet ... ne, Latin normally uses a subjunctive clause.

So:

• figit would mean he fastens/drives as a plain statement
• figere would be an infinitive, which is not the construction used here
• figat is the present subjunctive, which fits after ne

This kind of clause expresses what the father is warning the boy not to do, rather than stating what he actually does.

So the grammar is:

• monet
  • object + ne
    • subjunctive

That is why figat appears.

The subject of figat is understood to be the boy.

Latin often leaves out subject pronouns when they are clear from the context. Here:

• pater eum monet = the father warns him
• ne clavos male figat = that he not drive the nails badly

The he in figat is naturally the same person as eum, the person being warned.

So the father warns the boy not to do it badly.

Because clavos is the direct object of figat.

• figere means to fasten, to fix, or in this context to drive in
• the thing being driven in is the nails

So:

• clavi = nails as a subject
• clavos = nails as an object

In ne clavos male figat, the boy is the one doing the action, and the nails are what he would drive in badly.

Male is an adverb meaning badly or poorly.

It modifies figat, not clavos.

So:

• clavos male figat = he may drive the nails badly

It tells us how the action is done.

This is like English:

• He drives the nails badly

Latin adverbs often stand near the verb they modify, and male is placed before figat, which is very natural.

Because Classical Latin has no articles.

English says:

• the boy
• a hammer
• the father
• the nails

Latin simply says:

• puer
• malleum
• pater
• clavos

The listener figures out from context whether the sense is the, a, or just a general noun. That is normal in Latin.

Latin word order is more flexible than English word order, because the case endings show what each word is doing.

In this sentence:

• Puer is the subject of vult
• malleum is the object of tenere
• pater is the subject of monet
• eum is the object of monet
• clavos is the object of figat

Because those roles are marked by endings, the words do not have to stay in a rigid English-like order.

Still, the order here is quite natural:

• Puer malleum tenere vult = first idea
• sed pater eum monet = contrast with but
• ne clavos male figat = what the father warns him not to do

So the order is flexible, but not random.

Sed means but.

It connects the two main ideas:

• The boy wants to hold the hammer
• but the father warns him not to drive the nails badly

So sed shows a contrast:

• the boy wants to do something
• the father responds with caution or correction

It is a very common coordinating conjunction in Latin.

Yes, but monet ne ... figat is a very standard and idiomatic way to say it.

For example, Latin sometimes expresses commands, warnings, or advice in different ways, but here this construction is especially neat because it clearly means:

• warns him not to ...

So for a learner, the main takeaway is this pattern:

• moneo + accusative of person + ne + subjunctive

That is the key grammar to recognize in this sentence.