Magistra monet luxuriam bonos mores laedere.

It has two parts:

• Magistra monet = the main clause
• luxuriam bonos mores laedere = an indirect statement, giving the content of what the teacher says/warns

So the sentence is not just a simple subject-verb-object pattern. It is main verb + accusative-and-infinitive construction.

There is no separate word here for that.

In English, we often say The teacher warns that ....
In Latin, that idea is often expressed by an accusative + infinitive instead of a word meaning that.

So:

• luxuriam = the subject of the reported statement
• laedere = the verb of the reported statement

Together they mean something like that luxury harms ...

Because it is part of an indirect statement.

Latin usually changes the verb of an indirect statement into an infinitive. So instead of:

• Luxuria bonos mores laedit = Luxury harms good morals

you get:

• luxuriam bonos mores laedere = that luxury harms good morals

after a verb like monet.

Because in an indirect statement, the subject of the infinitive goes into the accusative case.

So in the direct statement:

• Luxuria is nominative, because it is the subject.

But in the indirect statement:

• luxuriam is accusative, because Latin uses the accusative for the subject of the infinitive.

This is often called the subject accusative.

They are doing two different jobs:

• luxuriam = the subject of laedere in the indirect statement
• bonos mores = the direct object of laedere

So both are accusative, but for different reasons.

That is completely normal in Latin accusative-and-infinitive constructions.

Because mores is masculine.

The noun is:

• mos, moris = custom, habit, moral practice
• plural mores = morals, character, customs

Since mores is masculine plural accusative here, the adjective must match it:

• bonos mores = masculine plural accusative

So bonos is correct.

Mores can mean things like:

• customs
• habits
• moral standards
• conduct
• character

In many contexts, boni mores means good morals or good conduct. So here bonos mores is a natural Latin expression for good morals.

Monet comes from moneo, monere, which can mean:

• remind
• warn
• advise
• urge

In this sentence, it has the sense of warning or advising. So Magistra monet ... means something like The teacher warns/advises that ...

The direct statement would be:

Luxuria bonos mores laedit.

Then after monet, Latin changes it into indirect statement form:

• Luxuria → luxuriam
• laedit → laedere

So:

• direct: Luxuria bonos mores laedit
• indirect: luxuriam bonos mores laedere

Not as much as it would be in English.

Latin word order is fairly flexible because the endings show the grammatical roles. The order here is natural and clear:

• Magistra monet first
• then the indirect statement

But Latin could move words around for emphasis. The endings still tell you what each word is doing.

They are:

• magistra, -ae = female teacher
• moneo, monere, monui, monitum = warn, advise, remind
• luxuria, -ae = luxury, excess, self-indulgence
• bonus, -a, -um = good
• mos, moris = custom, moral habit
• laedo, laedere, laesi, laesum = harm, injure

These dictionary forms help you see how the sentence forms are built.