Cum convivium finitum erit, puellae mensam ornatam spectabunt et cantabunt.

Because cum can be either:

• a preposition meaning with when it is followed by a noun in the ablative, or
• a conjunction meaning when, since, or although when it introduces a clause.

Here it introduces the clause cum convivium finitum erit, so it is a conjunction and means when.

Erit is the future of sum: he/she/it will be.

In this sentence, finitum erit means will have been finished. Latin often uses the future perfect in a time clause to show that one future action will be completed before another future action happens.

So the sequence is:

• first: convivium finitum erit = the banquet will have been finished
• then: puellae ... spectabunt et cantabunt = the girls will look ... and sing

English often uses simpler wording such as when the banquet is over, but Latin likes this clearer tense relationship.

Finitum erit is the future perfect passive indicative, third person singular, of finio.

It is made of:

• finitum = the perfect passive participle
• erit = future of sum

So literally it means it will have been finished.

Because the subject is convivium (banquet, neuter singular), the participle is also neuter singular: finitum.

Because finitum agrees with convivium.

Convivium is:

• nominative
• singular
• neuter

So the participle describing it must also be:

• nominative
• singular
• neuter

That is why it is finitum, not finitus or finita.

Here puellae is nominative plural, so it means the girls as the subject.

You can also tell from the verbs:

• spectabunt
• cantabunt

Both are third person plural, so they match puellae.

If puellae were, for example, dative singular or something else, the sentence structure would not fit these plural verbs nearly so naturally.

Because it is the direct object of spectabunt.

• mensa = table
• mensam = table as a direct object
• ornatam = decorated, agreeing with mensam

So:

• mensam is feminine singular accusative
• ornatam is also feminine singular accusative

The adjective has to agree with the noun it describes in gender, number, and case.

Because Latin specto commonly takes a direct object in the accusative.

So Latin says:

• mensam spectare = to look at the table

even though English usually needs the preposition at.

This is a very common difference between Latin and English: a Latin verb may take a direct object where English uses a preposition.

Both verbs end in -bunt, which is the future ending for third person plural in the 1st and 2nd conjugations.

So:

• spectabunt = they will watch/look at
• cantabunt = they will sing

Since both verbs are plural future forms, they match puellae and describe what the girls will do.

Not really. It goes directly with spectabunt, not with cantabunt.

So the structure is:

• puellae mensam ornatam spectabunt
• et cantabunt

In other words:

• the girls will look at the decorated table
• and will sing

The object belongs to the first verb. The second verb does not need an object.

Latin word order is much more flexible because endings show each word’s role.

In this sentence:

• cum convivium finitum erit comes first as the time clause
• then the main clause follows

That is a very natural Latin arrangement. Also, Latin often places an adjective after the noun, as in mensam ornatam, though other orders are possible too.

So the order may feel different from English, but the endings make the grammar clear.

Yes. Cum very often introduces subjunctive clauses, especially when it means things like:

• when in a background or circumstantial sense
• since
• although

But here the clause is a straightforward time clause about the future, so the indicative is used.

That is why we get cum convivium finitum erit, not a subjunctive form.

Yes. Very literally, it is:

When the banquet will have been finished, the girls will look at the decorated table and will sing.

That sounds awkward in English, but it helps show the Latin grammar:

• cum = when
• convivium finitum erit = the banquet will have been finished
• puellae = the girls
• mensam ornatam spectabunt = will look at the decorated table
• et cantabunt = and will sing

A more natural English translation usually smooths this out, but the literal version is useful for seeing how the Latin works.