Quidam pueri ad forum emendi causa eunt, alii autem audiendi causa, quia orator hodie de bono communi dicturus est.

Quidam means some, but with a slightly more definite feel than aliqui. It often suggests certain people within a group:

• quidam pueri = some boys / certain boys

Here it helps set up a contrast with alii:

• quidam pueri = some boys
• alii = others

So the sentence is dividing the boys into two groups.

This is a very common Latin pattern for contrasting two groups:

• quidam pueri ... eunt = some boys go ...
• alii autem = others, however / but others

Latin often avoids repeating everything if it is easy to understand. So in the second part, you mentally carry over the verb eunt from the first part.

In other words, the structure is basically:

• Some boys go to the forum for buying,
• but others go for listening.

Because ad takes the accusative when it means to or toward a place.

So:

• ad forum = to the forum

This is the normal construction for motion toward something.

Emendi causa is a standard Latin way to express purpose.

It consists of:

• emendi = the genitive gerund of emo (to buy)
• causa = for the sake of

Together:

• emendi causa = for the sake of buying, more naturally to buy

So Latin is literally saying something like for the sake of buying, where English would usually just say to buy.

Because when causa means for the sake of and expresses purpose, it regularly takes a genitive before it.

So:

• emendi causa = for buying
• audiendi causa = for hearing / listening

This is a fixed and very common Latin pattern.

Because Latin normally does not use the infinitive this way to express purpose.

English says:

• They go to buy
• They go to listen

But Latin often prefers a different structure, such as:

• emendi causa = for the sake of buying
• sometimes a purpose clause with ut
• sometimes a supine, depending on the verb

So emendi causa and audiendi causa are perfectly normal Latin ways to express purpose.

Grammatically it can cover either idea, since audio can mean hear or listen to, depending on context.

Here listening is the more natural sense, because the next clause mentions an orator who is going to speak. So the idea is that some boys are going in order to hear the speech or hear the speaker.

Yes, two things are understood rather than repeated.

First, eunt is understood from the first clause:

• quidam pueri ... eunt
• alii autem [eunt] audiendi causa

Second, the thing being heard is not stated explicitly. From the context, it is easy to understand something like:

• oratorem = the speaker
• or orationem = the speech

Latin often leaves out words that are obvious from context.

The most natural reading is that both groups go to the forum.

Latin often states a shared detail once and lets it apply to both halves of a contrast. So the sense is:

• some go to the forum to buy,
• others go to the forum to listen.

That fits the context, since the orator is apparently going to speak there.

Autem is a mild contrasting word. It often means:

• however
• but
• on the other hand

So alii autem means others, however or more naturally but others.

It helps mark the contrast between the two groups of boys.

Dicturus est is the future active participle of dico plus a form of sum.

Breakdown:

• dicturus = about to say / going to say
• est = is

Together:

• dicturus est = is going to say / is about to say

This is often called the active periphrastic. It can suggest future action, intention, or something expected to happen soon.

In this sentence, with orator, English will usually translate it more naturally as is going to speak.

Both can refer to the future, but they are not exactly the same.

• dicet = he will say / speak
• dicturus est = he is going to say / is about to say

The participle construction often feels a little more vivid or immediate. It can suggest that the event is anticipated or impending.

So here dicturus est fits well with hodie and with the idea that people are going to the forum specifically to hear him.

Yes, dico very often means say, but it can also be used in the sense of speak, deliver a speech, or make an address, especially in public-speaking contexts.

Since the subject is orator, translating it as speak is more natural in English.

So:

• orator ... dicturus est = the orator is going to speak

De takes the ablative, so both words are ablative singular:

• bono = ablative singular of bonum
• communi = ablative singular of commune

Together:

• de bono communi = about the common good

The phrase bonum commune means the common good. After de, it changes to the ablative:

• bonum commune → de bono communi

It most directly explains the situation just mentioned, especially the second group:

• the others are going for listening,
• because the orator is going to speak today about the common good.

But it can also color the whole sentence more broadly, since the public speech helps explain why the forum is busy and why some people are going there for that purpose.

So the strongest link is with audiendi causa, but it also fits the scene as a whole.

Usually classical Latin prefers the gerundive construction when the verbal idea has a direct object.

So instead of:

• librorum emendi causa

Latin often prefers:

• librorum emendorum causa = for the sake of buying books

In your sentence, there is no object stated, so the simple gerund works well:

• emendi causa
• audiendi causa