Magistra addit multos mercatores Delo olim convenisse.

It has two parts:

• Magistra addit = The teacher adds
• multos mercatores Delo olim convenisse = the statement being reported

So the sentence is:

• The teacher adds that many merchants once met on Delos.

This is a very common Latin pattern: a main verb of saying, thinking, knowing, etc., followed by an indirect statement.

Because after a verb like addit (she adds / says further), Latin usually uses accusative + infinitive for indirect statement.

So a direct statement would be:

• multi mercatores Delo olim convenerunt
  = many merchants met on Delos long ago

But after addit, Latin changes that into indirect statement:

• multos mercatores Delo olim convenisse
  = that many merchants had met / met on Delos long ago

So convenisse is not the main verb of the whole sentence; it is the infinitive inside the reported statement.

In a Latin indirect statement, the subject of the infinitive goes into the accusative.

So even though many merchants are the ones doing the meeting, Latin does not use nominative multi mercatores here. It uses:

• multos mercatores

That is because they are the subject of the infinitive convenisse inside indirect speech.

Also notice agreement:

• multos = masculine accusative plural
• mercatores = masculine accusative plural

The adjective matches the noun.

Convenisse is the perfect active infinitive of convenire.

In indirect statement, the tense of the infinitive is relative to the main verb:

• present infinitive = same time as the main verb
• perfect infinitive = earlier than the main verb
• future infinitive = later than the main verb

So with addit (she adds), convenisse means the meeting happened before the adding.

That is why English may translate it as:

• had met

But in smoother English, you may also see:

• met

depending on context.

Here convenire means:

• to come together
• to meet
• to assemble

So multos mercatores ... convenisse means many merchants met / gathered.

Be careful: convenire can mean other things in other contexts, such as to agree or to be suitable, but here the sense is clearly physical gathering, especially with Delo.

Delo means on Delos or at Delos.

Latin often treats names of towns and small islands differently from ordinary nouns of place. Instead of using a preposition like in, it can use the place name by itself.

So:

• Delo = at/on Delos

This is why Latin does not need in Delo here.

Olim usually means something like:

• once
• formerly
• long ago

In this sentence it most naturally means:

• long ago
• in former times

It modifies convenisse, telling us when the merchants met.

Because classical Latin normally does not use a conjunction meaning that after verbs of saying or thinking in this construction.

English says:

• The teacher adds that many merchants met...

Latin says:

• Magistra addit multos mercatores ... convenisse

So the idea of that is built into the accusative + infinitive construction. You understand it from the grammar, not from a separate word.

The direct statement would be:

• Multi mercatores Delo olim convenerunt.

That means:

• Many merchants met on Delos long ago.

When Latin turns that into indirect statement after addit, it changes:

• multi mercatores → multos mercatores
• convenerunt → convenisse

So:

• Magistra addit multos mercatores Delo olim convenisse.

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

Here the order is:

• Magistra = subject
• addit = main verb
• multos mercatores = subject of the infinitive
• Delo = place
• olim = time
• convenisse = infinitive

Latin often places the infinitive at or near the end of the indirect statement, so convenisse in final position is very natural.

English relies more on fixed word order, but Latin relies more on inflection.

Yes. Addit literally means she adds.

In context, it often works like:

• she adds
• she goes on to say
• she further says

So it is a normal reporting verb, just a little more specific than plain dicit.