Custos puerum ab igne prohibet.

The subject is custos.

You can tell because custos is in the nominative singular, the case normally used for the subject of a sentence. It means guard, watchman, or keeper.

So custos ... prohibet means the guard ... keeps away / prevents.

Puerum is the direct object of prohibet.

The verb prohibēre means to keep away, prevent, or protect from. In this sentence, the guard is acting on the boy, so the boy is the direct object.

• puer = boy
• puerum = boy in the accusative singular

So the sentence is about the guard keeping the boy away from something.

Because ab is a preposition that takes the ablative case, and igne is the ablative singular of ignis.

• ignis = fire
• igne = by/from the fire in the ablative
• ab = from, away from

So ab igne means from the fire or away from the fire.

In this sentence, prohibet ab igne expresses the idea of keeping someone away from danger.

Prohibet is the third person singular present active indicative of prohibeō, prohibēre.

It means:

• he/she/it keeps away
• he/she/it prevents
• he/she/it protects

In this sentence, it has the sense keeps the boy away from the fire or protects the boy from the fire.

So custos puerum ab igne prohibet can be understood as:

• The guard keeps the boy away from the fire
• The guard protects the boy from the fire

Latin has no definite article like English the, and no indefinite article like a/an.

So:

• custos can mean guard, a guard, or the guard
• puerum can mean boy, a boy, or the boy

You decide from context which English article is best.

Yes. Latin word order is much freer than English word order because the endings show each word’s role.

So all of these could mean essentially the same thing:

• Custos puerum ab igne prohibet
• Puerum custos ab igne prohibet
• Ab igne puerum custos prohibet

The usual sentence you were given is perfectly normal, but Latin can move words around for emphasis, style, or rhythm.

English depends much more on word order, while Latin depends more on case endings.

Both a and ab mean from and take the ablative.

A common guideline is:

• ab is used especially before a vowel or h
• a is often used before a consonant

Since igne begins with a vowel sound, ab igne is the expected form.

So ab igne = from the fire.

The nouns in this sentence come from different declensions:

• custos, custodis = guard → third declension
• puer, puerī = boy → second declension
• ignis, ignis = fire → third declension

Their forms here are:

• custos = nominative singular
• puerum = accusative singular
• igne = ablative singular

This is a good example of how Latin mixes words from different declensions in one sentence.

Yes, in this sentence it is naturally understood as masculine, since it refers to the guard as the one acting.

But custos is a noun that can be used for a guardian/guard/keeper more generally, and its grammatical gender can depend on the person being described.

Here, the important thing for understanding the sentence is not so much gender as the fact that custos is the subject.

Yes. Prohibeō is commonly used with:

• a direct object: the person or thing being kept away
• often ab or ā
  • ablative: the danger or thing avoided

So in this sentence:

• puerum = the one being kept away
• ab igne = what he is being kept away from

That is a very natural Latin pattern: someone + ab something + prohibēre

Here is a full breakdown:

• Custos — noun, nominative singular, subject: guard
• puerum — noun, accusative singular, direct object: boy
• ab — preposition taking the ablative: from, away from
• igne — noun, ablative singular of ignis: fire
• prohibet — verb, third person singular present active indicative of prohibeō: keeps away / prevents / protects

So the structure is:

Subject + direct object + prepositional phrase + verb

That gives the sense: The guard keeps the boy away from the fire.