Magistra docet gravitatem meliorem esse quam levitatem, si de rebus publicis disputamus.

The sentence has three parts:

1. Main clause: Magistra docet = The teacher teaches / The teacher says
2. Indirect statement: gravitatem meliorem esse quam levitatem = that seriousness is better than frivolity
3. Conditional clause: si de rebus publicis disputamus = if we are discussing public affairs

So the full structure is:

The teacher teaches [that seriousness is better than frivolity], if we are discussing public affairs.

More naturally in English: The teacher teaches that seriousness is better than frivolity when discussing public affairs.

This is one of the most important Latin constructions for English speakers to learn: the accusative-and-infinitive construction, often used for indirect statements.

The direct statement would be:

Gravitas melior est quam levitas.
Seriousness is better than frivolity.

But after a verb like docet, Latin usually changes that into indirect statement:

• gravitas → gravitatem
• melior est → meliorem esse
• levitas → levitatem

So:

gravitatem meliorem esse quam levitatem

In other words, the subject of the infinitive is put into the accusative in Latin.

Because meliorem agrees with gravitatem.

• gravitatem is accusative singular feminine
• so the comparative adjective modifying it must also be accusative singular feminine
• therefore: meliorem

If this were a direct statement, you would have:

Gravitas melior est
where melior is nominative, agreeing with gravitas

But in indirect statement:

gravitatem meliorem esse
both words shift into the accusative.

Because Latin uses an infinitive in indirect statement, not a finite verb.

Compare:

• Direct statement: Gravitas melior est quam levitas.
• Indirect statement: Magistra docet gravitatem meliorem esse quam levitatem.

English often uses that + a normal verb:

• The teacher teaches that seriousness is better than frivolity.

Latin usually prefers:

• [accusative subject] + [infinitive]

So esse is exactly what we expect here.

Because Latin often does not use a separate word meaning that in this kind of sentence.

In English, we say:

• The teacher teaches that seriousness is better than frivolity.

In Latin, that idea is expressed by the accusative-and-infinitive construction instead of a that-clause:

• Magistra docet gravitatem meliorem esse quam levitatem.

So the combination gravitatem meliorem esse is doing the job that that seriousness is better does in English.

Quam means than in comparisons.

Here the comparison is:

gravitatem meliorem esse quam levitatem
seriousness to be better than frivolity

Since gravitatem is accusative in the indirect statement, the thing compared with it, levitatem, also appears in the same case after quam.

So this is completely normal:

• gravitatem ... quam levitatem

You may also learn that Latin sometimes uses the ablative of comparison instead of quam, but quam + same case is very common and very clear.

Yes, in many contexts Latin can compare something with a comparative adjective by using either:

• quam + same case
• or the ablative of comparison

So the idea could potentially be expressed with an ablative comparison. But in this sentence, quam levitatem is perfectly standard and especially clear.

For a learner, it is often easiest to recognize:

• meliorem ... quam ... = better than ...

Because this is a simple, real condition: if we are discussing public affairs.

Latin commonly uses:

• si + indicative for a straightforward condition
• si + subjunctive in more special or less straightforward kinds of conditions

Here the speaker is not saying something hypothetical or contrary to fact. It is more like a general rule:

If we are discussing public affairs, seriousness is better than frivolity.

So disputamus is exactly what we would expect.

Because the preposition de takes the ablative.

So:

• de = about / concerning
• rebus publicis = ablative plural

The phrase res publica literally means the public thing, but in practice it means things like:

• the state
• the republic
• public affairs
• politics

In the plural, de rebus publicis often means about public affairs or about political matters.

Not in the physical sense.

Here gravitas means something like:

• seriousness
• dignity
• weightiness
• sense of responsibility

And levitas means the opposite sort of quality:

• light-mindedness
• frivolity
• superficiality
• lack of seriousness

So the contrast is moral or social, not physical.

Latin often uses the first person plural in a broad, general way, much like English we.

So si de rebus publicis disputamus can mean:

• if we are discussing public affairs
• if one is discussing public affairs
• when people discuss public affairs

It makes the statement feel more inclusive and general.

Latin word order is more flexible than English word order because the endings show the grammatical relationships.

So this order:

Magistra docet gravitatem meliorem esse quam levitatem, si de rebus publicis disputamus.

is perfectly natural, but Latin could rearrange parts for emphasis.

Still, the current order is helpful:

• Magistra docet sets up the main action first
• then comes the content of what is taught
• then the si clause gives the condition

Also, placing meliorem near gravitatem helps show what adjective goes with what noun.

The direct statement would be:

Gravitas melior est quam levitas, si de rebus publicis disputamus.

Then, after magistra docet, Latin changes it into indirect statement:

• gravitas → gravitatem
• melior → meliorem
• est → esse
• levitas → levitatem

So:

Magistra docet gravitatem meliorem esse quam levitatem, si de rebus publicis disputamus.

This is a very useful transformation to practice, because it appears constantly in Latin prose.