Opinio matris est bonos magistratus bonum commune, non commodum privatum, quaerere debere.

Matris is the genitive singular of mater, meaning of the mother.

So:

• opinio = opinion
• matris = of the mother

Together, opinio matris means the mother’s opinion or the opinion of the mother.

This is a very common Latin way to show possession or association: a noun + a genitive noun.

The main clause is:

• Opinio matris est = It is the mother’s opinion / The mother’s opinion is

After that comes an indirect statement:

• bonos magistratus ... quaerere debere

So the sentence is built like this:

• Opinio matris est
  • [accusative + infinitive construction]

In other words:

• The mother’s opinion is that good magistrates ought to seek the common good, not private advantage.

This is a standard Latin pattern after nouns or expressions involving belief, judgment, report, and so on.

Because they are the subject of the infinitive in an indirect statement.

In English, we say:

• The mother’s opinion is that good magistrates ought to seek...

In Latin, after expressions like opinio est, the content of the opinion is often expressed with accusative + infinitive:

• bonos magistratus = the subject of the infinitive phrase
• quaerere debere = ought to seek

So even though good magistrates would be the logical subject of seek, Latin puts that subject into the accusative in this construction.

By itself, magistratus could look ambiguous, because fourth-declension masculine nouns often have the same form for nominative plural and accusative plural.

But here the adjective bonos makes it clear:

• bonos is accusative plural masculine
• therefore magistratus must also be accusative plural

So bonos magistratus definitely means good magistrates as an accusative phrase.

Because debere can take another infinitive.

• quaerere = to seek
• debere = to owe, and by extension to ought, should, must
• quaerere debere = to ought to seek / more naturally, ought to seek

Then, because the whole reported idea is in an indirect statement, debere itself appears as an infinitive:

• bonos magistratus ... quaerere debere
• that good magistrates ought to seek...

So the first infinitive (quaerere) depends on debere, and the second infinitive (debere) belongs to the accusative-and-infinitive construction.

Because Latin often does not use a conjunction like English that in this kind of sentence.

English says:

• The mother’s opinion is that good magistrates ought to seek...

Latin instead uses the accusative + infinitive construction:

• opinio matris est bonos magistratus ... debere

So the idea of English that is built into the grammar of the accusative-and-infinitive construction, rather than being expressed by a separate word.

They are both accusative singular neuter because they are the direct objects of quaerere.

• quaerere = to seek
• seek what?
  • bonum commune = the common good
  • commodum privatum = private advantage

So the structure is:

• bonos magistratus = subject of the infinitive, in the accusative
• bonum commune = object of quaerere
• commodum privatum = contrasting object after non

Both word orders are possible in Latin. Latin word order is more flexible than English.

Here, bonum commune means the common good:

• bonum = good, here functioning like a noun: a good or the good
• commune = common, agreeing with bonum

Latin often places words for emphasis or rhythm rather than following a fixed English-like order. So bonum commune is perfectly natural.

The same applies to commodum privatum:

• commodum = advantage, benefit
• privatum = private

Originally it is an adjective, but here it is being used substantively, meaning it functions like a noun.

So:

• bonum literally comes from good
• but in context it means a good or the good

Thus:

• bonum commune = the common good

This is very common in Latin: adjectives can act like nouns when the meaning is clear.

Here debere expresses obligation, so it can be translated in several similar ways:

• ought to seek
• should seek
• must seek

The best English choice depends on tone. In a moral or political statement like this, ought to seek is often a very good translation because it preserves the sense of duty without sounding too harsh.

Because the contrast is between the two things being sought:

• bonum commune
• non commodum privatum

So the sense is:

• the common good, not private advantage

The non directly negates or excludes commodum privatum. This placement makes the contrast especially clear.

It is not mainly saying they ought not to seek; it is saying what they ought to seek is the common good, not private advantage.

In some contexts Latin can use other constructions, but after a noun like opinio with est, the accusative + infinitive is a very normal and classical way to express the content of the opinion.

So:

• Opinio matris est bonos magistratus ... debere

is a straightforward way to say:

• The mother’s opinion is that good magistrates ought to...

A learner should recognize this as a very common reported-thought or reported-statement structure.

That is just a different way of expressing the idea.

Compare:

• Mater putat bonos magistratus... debere = The mother thinks that good magistrates ought...
• Opinio matris est bonos magistratus... debere = It is the mother’s opinion that good magistrates ought...

The sentence you have is slightly more formal or abstract in style because it focuses on the opinion itself rather than directly on the mother as the thinker.

There are several important agreement patterns:

• bonos magistratus

  • bonos agrees with magistratus
  • both are masculine accusative plural

• bonum commune

  • commune agrees with bonum
  • both are neuter accusative singular

• commodum privatum

  • privatum agrees with commodum
  • both are neuter accusative singular

So even though Latin word order is flexible, agreement helps you see which words belong together.

A fairly literal unpacking would be:

• Opinio matris est = The opinion of the mother is
• bonos magistratus = good magistrates
• bonum commune = the common good
• non commodum privatum = not private advantage
• quaerere debere = to ought to seek

Putting that into natural English:

• The mother’s opinion is that good magistrates ought to seek the common good, not private advantage.

That kind of literal breakdown is useful because it shows the Latin grammar, even if the final English needs smoothing out.