Post lectionem magistra discipulos ad forum ducit, ubi contio magna iam ante rostra convenit.

Because post is a preposition that takes the accusative case when it means after.

• post lectionem = after the lesson
• lectio is a 3rd-declension noun
• nominative: lectio
• accusative: lectionem

So this is a very normal pattern:

• post cenam = after dinner
• post bellum = after the war
• post lectionem = after the lesson

Magistra is in the nominative singular, which is the usual case for the subject of the verb.

Here the main verb is ducit = leads or is leading, and magistra is the one doing that action.

So:

• magistra = the teacher
• ducit = leads
• therefore magistra is the subject

Also, magistra is feminine, so it means female teacher.

Because discipulos is the direct object of ducit.

The verb ducere means to lead. The person being led goes in the accusative:

• magistra discipulos ducit = the teacher leads the students

So:

• magistra = subject
• discipulos = object
• ducit = verb

Discipulos is the accusative plural of discipulus.

Latin often uses ad + accusative to show motion toward a place.

So:

• ad forum = to the forum

This is different from being in a place:

• in foro = in the forum

A useful contrast:

• ad forum ducit = leads to the forum
• in foro est = is in the forum

So ad makes the direction very clear.

Ubi here means where and introduces a relative/adverbial clause of place.

The sentence structure is:

• Post lectionem magistra discipulos ad forum ducit
• ubi contio magna iam ante rostra convenit

So ubi connects the forum with what is happening there:

• to the forum, where a large crowd is already gathering/has gathered in front of the rostra

It is not asking a question here. It is simply describing the place.

Because contio is a singular noun meaning something like assembly, meeting, or crowd.

Even though many people are involved, Latin treats the word itself as grammatically singular:

• contio magna = a large crowd / a large assembly

That is why the verb is also singular:

• contio ... convenit

This is similar to English expressions like the crowd is gathering, where crowd is singular in form.

Because it agrees with contio.

Latin adjectives agree with the nouns they describe in:

• gender
• number
• case

Here:

• contio is feminine singular nominative
• so the adjective must also be feminine singular nominative
• therefore: magna

So:

• contio magna = a large assembly/crowd

Here convenit comes from convenire, which can mean:

• come together
• assemble
• gather
• sometimes meet

So in this sentence it means that the crowd is gathering or has gathered.

Do not confuse this with the English-derived sense it is convenient. Although Latin convenit can sometimes mean it is fitting/appropriate, that is not the meaning here. Here the context is clearly physical gathering:

• contio ... convenit = the crowd gathers / has assembled

Iam means already or now by this time.

It adds the idea that when the teacher leads the students to the forum, the assembly is already there or already in the process of gathering.

So:

• contio magna iam ... convenit = a large crowd has already gathered or is already gathering

It gives a time nuance: the event in the forum is already underway.

Ante is a preposition meaning before or in front of, and it takes the accusative case.

So:

• ante rostra = in front of the rostra

Rostra is neuter plural. In Roman context, the rostra was the speakers' platform in the forum.

So the phrase means the crowd is gathering:

• before/in front of the speakers' platform

This is a good example of a place name or monument that is grammatically plural in Latin.

In Roman public life, the rostra was a public platform in the forum from which speeches were given.

So ante rostra suggests a political or public setting:

• an assembly or crowd gathers in front of the platform
• probably to hear a speaker or take part in public business

For a learner, it helps to think of rostra as:

• the speaker's platform
• the public speaking stage in the forum

Latin word order is much freer than English word order. End position is often used for emphasis or simply as a natural place for the verb.

In the main clause:

• Post lectionem magistra discipulos ad forum ducit

In the subordinate clause:

• ubi contio magna iam ante rostra convenit

Putting convenit at the end is very typical Latin style. It allows the sentence to build up the scene first:

• where
• a large assembly
• already
• in front of the rostra
• gathers/has gathered

English depends much more on fixed word order, but Latin uses endings to show grammatical roles, so the order can be more flexible.

All three can be possible depending on context.

Latin present tense often overlaps with several English possibilities:

• gathers
• is gathering
• sometimes, in context, even has gathered if the action is seen as already accomplished

Because of iam, many readers feel:

• has already gathered or
• is already gathering

The exact English choice depends on how the translator wants the scene to sound. The Latin itself leaves a little flexibility.

In Latin, ubi can sometimes mean when, but here where is much more natural.

Why? Because the previous phrase gives a place:

• ad forum = to the forum

So ubi naturally refers back to that place:

• to the forum, where ...

If it meant when, the sentence would feel less straightforward in this context. The place connection makes where the best reading.