In foro mercatores sine ordine sedentes pretia magna clamant.

Because in can take different cases depending on its meaning:

• in + ablative = in / on a place, showing location
• in + accusative = into / onto a place, showing motion toward

Here the merchants are already in the forum, not moving into it, so Latin uses the ablative:

• forum = nominative/accusative singular
• foro = ablative singular

So in foro means in the forum / marketplace.

Mercatores is nominative plural, and it is the subject of clamant.

You can tell this because:

• mercator = merchant
• nominative plural = mercatores
• clamant means they shout, so it needs a plural subject
• mercatores matches that perfectly: the merchants shout

Latin often relies on case endings, not word order, to show the subject.

Sine is a preposition meaning without, and it takes the ablative case.

So:

• ordo = order
• ordine = ablative singular
• sine ordine = without order

This phrase describes the way the merchants are sitting: they are sitting without order, that is, in a disorderly or unorganized way.

Sedentes is a present active participle, from sedere meaning to sit.

A participle is a verbal adjective. Here it describes mercatores:

• mercatores = the merchants
• sedentes = sitting

So the Latin means something like:

• the merchants, sitting without order, shout...
• or the merchants who are sitting without order shout...

If Latin used sedent, that would make it another main verb: the merchants sit and shout.
But sedentes makes the sitting a description of the merchants rather than a separate main action.

Because sedentes agrees with mercatores in case, number, and gender.

• mercatores = nominative plural masculine
• sedentes = nominative plural masculine/feminine participle

That agreement shows that sedentes is modifying mercatores.

Latin adjectives and participles usually make their relationships clear by agreement rather than position.

Pretia magna is accusative plural, and it is the direct object of clamant.

Breakdown:

• pretium = price
• pretia = accusative plural (also nominative plural, but here it is object)
• magnus, magna, magnum = great / large
• magna agrees with pretia, so it is also accusative plural neuter

So the merchants are shouting high/great prices.

Because pretia is neuter plural, and the adjective must agree with it.

• pretium is a neuter noun
• its plural is pretia
• the neuter plural form of magnus is magna

So:

• masculine plural: magni
• feminine plural: magnae
• neuter plural: magna

Since pretia is neuter plural, magna is the correct form.

Clamant is the 3rd person plural present active of clamare:

• clamo = I shout
• clamant = they shout

In this context it means something like:

• they shout out
• they call out
• they cry aloud

With merchants in a marketplace, clamant pretia magna suggests they are loudly calling out prices.

Yes. That is a very common and natural way to translate a participle into English.

So:

• mercatores sedentes can mean the merchants sitting
• or the merchants who are sitting

English often prefers a relative clause where Latin uses a participle.

Latin word order is much more flexible than English word order because Latin uses case endings to show grammar.

This sentence is arranged roughly like this:

• In foro = place
• mercatores = subject
• sine ordine = adverbial phrase
• sedentes = participle describing the subject
• pretia magna = object
• clamant = verb

A very natural English order might be:

• In the forum, merchants sitting without order shout high prices.

Or more idiomatically:

• In the forum, merchants sit in disorder and shout high prices.

Latin often puts the main verb near the end, but that is a tendency, not a strict rule.

Literally, magna means great or large. But with prices, English usually says high prices.

So although the Latin is literally close to great prices, the natural English sense is:

• high prices
• large prices only in a very literal or awkward sense

This is a good reminder that adjectives do not always match perfectly word-for-word across languages.

Because classical Latin has no articles.

Latin does not have separate words for:

• the
• a / an

So mercatores can mean:

• merchants
• the merchants

And in foro can mean:

• in a forum
• in the forum

The correct English article is chosen from context.

Yes. It works as a descriptive phrase modifying mercatores.

You can think of it as:

• mercatores = the merchants
• sine ordine sedentes = sitting without order

So the whole subject phrase is:

• the merchants sitting without order

Latin often packs a lot of information into participial phrases like this.

They are:

• in = in, into
• forum, fori = forum, marketplace
• mercator, mercatoris = merchant
• sine = without
• ordo, ordinis = order, arrangement
• sedeo, sedere = sit
• pretium, pretii = price
• magnus, magna, magnum = great, large
• clamo, clamare = shout

Knowing the dictionary form helps you see how the forms in the sentence were built.