Orator ad rostrum ascendit et de bono communi loquitur.

Because orator is in the nominative singular, the case normally used for the subject of a finite verb in Latin.

So in this sentence:

• orator = the speaker / orator
• it is the thing that does both actions:
  • ascendit
  • loquitur

Latin often leaves the subject in a simple nominative form without needing a separate word like English he unless emphasis is wanted.

Rostrum is accusative singular because it follows the preposition ad.

A very common rule is:

• ad
  • accusative = to / toward

So:

• ad rostrum = to the platform / to the rostrum

This expresses motion toward a place.

Here rostrum means a speaking platform or public platform.

Historically, the Rostra in Rome was a famous speakers' platform in the Forum. So in Latin, rostrum can refer to a platform from which someone speaks publicly.

So ad rostrum ascendit gives the picture of a speaker going up onto a platform before speaking.

By form alone, ascendit can be ambiguous.

It can be:

• present: he/she ascends
• or perfect: he/she ascended / has ascended

This happens with some Latin verbs.

In this sentence, because it is followed by loquitur in the present, the most natural understanding is present-time narration:

• The speaker ascends the platform and speaks...

In some contexts, though, ascendit could definitely be past.

Because loquor, loqui, locutus sum is a deponent verb.

A deponent verb:

• has passive forms
• but an active meaning

So loquitur looks passive in form, but it means:

• he/she speaks
• not he/she is spoken

This is a very common thing for Latin learners to notice. It feels strange at first, but after a while you start recognizing deponent verbs quickly.

Because the preposition de takes the ablative.

So:

• de
  • ablative = about / concerning / on the subject of

Here:

• bono is ablative singular
• communi is also ablative singular

They go together as:

• de bono communi = about the common good

The adjective communi agrees with bono in:

• case
• number
• gender

It is an ablative singular phrase built from:

• bonum = good, benefit, good thing
• communis, commune = common, shared, public

Together, bonum commune means the common good.

After de, it becomes ablative:

• de bono communi

So this is not two unrelated words; it is one unit meaning about the common good.

Yes. Latin adjective order is more flexible than English word order.

So both of these are possible:

• de bono communi
• de communi bono

The meaning is basically the same.

That said, bonum commune is a very familiar and idiomatic expression for the common good, so de bono communi sounds very natural.

Because Classical Latin does not have definite or indefinite articles like English.

So Latin often simply says:

• orator rather than the speaker or a speaker
• ad rostrum rather than to the platform
• de bono communi rather than about the common good

English has to choose the or a/an, but Latin usually leaves that to context.

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

In this sentence:

• orator tells you the subject
• rostrum is marked by ad
• bono communi is marked by de
• the verb endings show who is acting

So Latin does not need a rigid English-style order.

This sentence is actually quite natural Latin:

• subject first: orator
• then a phrase of movement: ad rostrum
• then the verb: ascendit
• then et
• then another phrase: de bono communi
• then the second verb: loquitur

Latin often likes placing verbs toward the end, though many other orders are possible too.

Because the same subject carries over naturally to the second verb.

So Latin understands:

• Orator ascendit
• et [orator] loquitur

The second orator is unnecessary because it is obvious that the same person is doing both actions.

English does this too sometimes:

• The speaker goes up to the platform and speaks...

We do not have to repeat the speaker before speaks.

Et simply means and.

It joins the two actions of the same subject:

• ascendit = goes up / ascends
• loquitur = speaks

So the structure is:

• The speaker ascends the platform
• and
• speaks about the common good

It is a straightforward coordinating conjunction.