Turba forum complet, cum regina ad templum ambulat.

Because turba is a singular noun. In Latin, as in English, a collective noun can refer to a group but still be grammatically singular.

• turba = crowd
• It is nominative singular here, so the verb is also singular: complet = fills

So Latin treats the crowd as one grammatical unit, even though it is made up of many individuals.

You can tell from both meaning and grammar.

• complet means fills, and fill normally takes a direct object: someone fills something.
• Here, the thing being filled is forum.

So:

• turba = subject
• complet = verb
• forum = direct object

The direct object is normally in the accusative case in Latin.

Because forum is a neuter second-declension noun, and in neuter nouns the nominative and accusative singular are often identical.

So:

• nominative singular: forum
• accusative singular: forum

That means you cannot identify its case by the ending alone here. Instead, you use its role in the sentence:

• turba is doing the action
• forum is receiving the action

Therefore forum is accusative in function, even though its form looks the same as the nominative.

No. Here cum means when or while, not with.

Latin cum has two common uses:

• as a preposition: with
• as a conjunction: when, while, sometimes since

In this sentence, cum is followed by a whole clause:

• regina ad templum ambulat

That shows it is a conjunction, not a preposition.

If it meant with, you would expect cum to be followed by a noun in the ablative, not by a full clause with a verb.

Because regina is the subject of ambulat.

In the clause:

• regina ad templum ambulat

the queen is the one doing the walking, so regina is nominative.

A learner often expects cum to force a special case, but that only happens when cum is a preposition meaning with. Here cum is a conjunction introducing a subordinate clause, so regina simply takes the case required by its job in that clause: nominative subject.

Because ad shows movement toward a place.

• ad
  • accusative = to, toward
• templum is accusative because ad takes the accusative

So ad templum ambulat means walks to the temple or walks toward the temple.

Without ad, the meaning would be different and much less straightforward. Latin often uses ad to mark motion toward a destination.

Because the preposition ad takes the accusative case.

So:

• ad = to, toward
• templum = accusative singular of templum

This is a very common Latin pattern:

• ad urbem = to the city
• ad villam = to the house/farm
• ad templum = to the temple

Because the sentence is describing an action and a time relationship in the present.

• complet = fills
• ambulat = walks

With cum + indicative, Latin often gives a straightforward temporal sense such as when or while.

Depending on context, English may translate this in more than one natural way:

• The crowd fills the forum when the queen walks to the temple
• The crowd fills the forum while the queen is walking to the temple

Latin present tense is often slightly broader than simple English present, so English may use either a simple or progressive form depending on what sounds natural.

Because classical Latin does not have definite or indefinite articles like English the or a/an.

So a noun like turba can mean:

• a crowd
• the crowd

and regina can mean:

• a queen
• the queen

Context tells you which is meant. In many teaching sentences, the translation uses the because that is the most natural English wording.

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

This sentence could also appear as:

• Cum regina ad templum ambulat, turba forum complet.

That would still mean essentially the same thing.

The version you have places the main clause first and the cum clause second. Latin authors often move words around for emphasis, rhythm, or style. Even so, the endings still show the relationships:

• turba = subject of complet
• forum = object of complet
• regina = subject of ambulat
• ad templum = direction

So word order matters less in Latin than in English, though it can still affect emphasis.