Mater filium monet ne officium suum differat.

Because mater is the subject of the main verb monet.

• mater = mother
• nominative singular
• it answers the question Who is warning?

So in Mater filium monet, mater is the one doing the action.

Because filium is the direct object of monet.

• filium = son
• accusative singular of filius
• it answers the question Whom does the mother warn/admonish?

So:

• mater = the one who warns
• filium = the person warned

Monet is from moneo, monere, and it can mean things like:

• warns
• advises
• reminds
• urges

In this sentence, warns or advises fits best.

So Mater filium monet... means The mother warns/advises her son...

Here ne means that ... not or simply not, introducing a negative clause after a verb like monet.

So:

• Mater filium monet ne... = The mother warns her son not to...

This is a very common Latin pattern:

• verb of warning/advising
• followed by ne
• followed by a subjunctive verb

In other words, ne is what makes the warning negative.

Because after a verb like monet, Latin commonly uses ne + subjunctive to express what someone is being warned not to do.

So this is not just a plain statement like he does not delay.
It is part of the warning:

• monet ne differat = warns him not to delay

That is why Latin uses differat, the present subjunctive, rather than differt, the present indicative.

Differat is:

• 3rd person singular
• present subjunctive
• active
• from differo, differre, distuli, dilatum

So it means he may delay / he should delay in form, but in this sentence, because of monet ne, it means:

• that he delay not
• more naturally, not to delay

The understood subject of differat is he, referring to filium.

The subject is understood from the context: filium.

Latin often leaves subject pronouns unstated because the verb ending already gives the person and number.

• differat = he may delay
• that he is the son

So the clause means:

• ne officium suum differat = that he should not delay his duty

Officium is the direct object of differat, so it is in the accusative singular.

• officium = duty, service, obligation
• suum agrees with officium in case, number, and gender:
  • accusative
  • singular
  • neuter

So:

• officium suum = his duty

Yes. Suus, sua, suum is a reflexive possessive adjective, often meaning his own, her own, or their own.

In this sentence, suum refers to the subject of the clause it is in. The subject of differat is the son, so:

• officium suum differat = he delay his own duty
• in natural English: delay his duty

Latin uses suus when the possession belongs to the subject of that clause.

Because suum refers back to the subject of its own clause, while eius would refer to someone else.

Here the subject of differat is the son, so:

• officium suum = his own duty (the son’s duty)

If Latin said officium eius, that would normally mean his duty where his refers to some other male person, not the subject of differat.

So suum is exactly the right choice here.

A learner might first think that, because ne + subjunctive often introduces a negative purpose clause. But here it is better understood as the clause used after a verb of warning/advising.

So rather than:

• She warns him in order that he not delay...

the real sense is:

• She warns him not to delay...

In other words, this is the standard construction after moneo with a negative idea.

Yes. Differre here means to put off, postpone, or delay. It is related to the English word defer.

So:

• officium suum differat = he may postpone his duty
• more naturally: he should not put off his duty

Be careful not to confuse this with English differ meaning be different. In this Latin sentence, the meaning is delay/postpone.

Latin word order is more flexible than English word order because the endings show each word’s function.

So Mater filium monet ne officium suum differat could be rearranged in several ways without changing the basic meaning.

Still, the given order is natural and clear:

• Mater first: the topic or subject
• filium monet: the main action
• ne officium suum differat: what she warns him not to do

Often Latin puts the verb at the end of a clause, and here differat comes at the end of the subordinate clause in a very typical way.

Yes. Several translations are possible, depending on how literally or naturally you want to render it:

• The mother warns her son not to delay his duty.
• The mother advises her son not to put off his duty.
• The mother admonishes her son not to postpone his duty.

All of these reflect the Latin well. The exact English wording depends on how strongly you want to translate monet and differat.