Mater puerum monet ne retro in scalis cadat.

The case endings tell you.

• mater is nominative singular, the normal case for the subject.
• puerum is accusative singular, the normal case for a direct object.

So mater is the one doing the warning, and puerum is the person being warned.

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

The verb moneo, monere can have a range of meanings in Latin, including:

• warn
• advise
• remind
• admonish

In this sentence, because it is followed by ne ... cadat, the sense is clearly warns the boy not to fall.

Because ne is the normal word used to introduce a negative idea in this kind of subordinate clause.

Latin often uses:

• non to negate a statement: he does not fall
• ne with the subjunctive for ideas like not to, lest, or that ... not

After a verb like monet, Latin says:

• monet ne cadat = warns him not to fall

So ne is not just a simple not; it introduces the whole negative clause.

Because cadat is inside a clause introduced by ne after a verb of warning.

This clause does not state a plain fact. It expresses what the mother wants to prevent. In English, we often translate that idea with not to fall or that he should not fall.

So:

• cadit would mean he falls as a statement of fact
• cadat means that he fall / should fall, in a dependent subjunctive clause

The present subjunctive fits because the main verb is present (monet) and the falling is still potential or future from that point of view.

The mother is warning him now about something that might happen afterward.

A very simple way to think of it is:

• monet ne cadat = she warns him not to fall
• if the main verb were past, you would often expect caderet instead: monuit ne caderet = she warned him not to fall

So the tense of the subjunctive helps show its time relationship to the main verb.

It is a subordinate clause depending on monet.

A beginner-friendly way to understand it is:

• monet = warns
• ne ... cadat = not to fall

So the whole thing works like:

• The mother warns the boy not to fall backward on the stairs.

In more grammatical terms, it is a subjunctive clause after a verb of warning/advising.

Retro is an adverb meaning back, backward, or backwards.

Here it modifies cadat, so the idea is:

• fall backward
• fall backwards

It does not describe scalis. It is about the direction of the fall.

Two things are going on here.

First, scalis is ablative plural from scalae, which usually means stairs or steps. Latin often uses the plural here, just as English commonly says stairs.

Second, in with the ablative often means in or on a place:

• in scalis = on the stairs / on the stairway

So the phrase tells you where the danger is.

Because this sentence is describing the fall as happening on the stairs, not focusing on movement down from them.

• in scalis emphasizes location: on the stairs
• de scalis would suggest down from the stairs or off the stairs

Latin chooses prepositions based on the exact idea being expressed. Here the sentence is centered on the dangerous location.

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

So:

• mater can mean mother or the mother
• puerum can mean a boy or the boy

You figure out which is natural from context. In this sentence, English normally uses the mother and the boy, but Latin does not need separate words for that.

It matters for emphasis and style, but not as much as in English.

This sentence has a very normal Latin structure:

• Mater = subject
• puerum = object
• monet = verb
• then the ne clause

Because the case endings already show the roles of the nouns, Latin can move words around more freely than English can. But this order is clear and natural.

By itself, monet can sometimes mean reminds. But in this sentence, warns is the better meaning.

That is because ne cadat shows that the mother is trying to prevent something from happening. So the sense is not just bringing something to mind, but cautioning him.

No.

• retro means backward/back
• in scalis means on the stairs

So retro is not functioning like an English preposition such as behind. It is an adverb describing how he might fall: backward on the stairs.