Vespere, postquam omnes convenerint, vespertinum carmen in atrio audietur.

Vespere means in the evening or at evening. It is an ablative form used to express time when, and Latin often does this without a preposition.

So instead of saying in the evening, Latin can simply say vespere. This is very common with words of time.

Yes, they are related, but they do different jobs.

• vespere = in the evening; it tells when something happens
• vespertinum = evening as an adjective; it describes carmen

So vespertinum carmen means the evening song, while vespere sets the time: in the evening.

Postquam means after. It introduces a subordinate clause: postquam omnes convenerint = after everyone has gathered.

This clause tells us what must happen before the main action: the song will be heard after everyone has gathered.

In this sentence, convenerint is best understood as future perfect indicative: they will have gathered.

The tricky part is that the form convenerint can also be perfect subjunctive in other contexts. Here, though, because the main verb is future (audietur) and the meaning is after everyone has gathered, the future perfect makes best sense.

Latin often uses the future perfect in a subordinate clause when that action will be completed before another future action.

So the sequence is:

• first: everyone will have gathered
• then: the evening song will be heard

English sometimes does something similar with after everyone has gathered, even though we do not usually call that a future perfect in normal English grammar explanation.

The subject is omnes, meaning all or everyone.

Here omnes is being used substantively, meaning all the people rather than modifying a noun that is stated explicitly. Since convenerint is plural, omnes must mean all the people / everyone.

The subject is vespertinum carmen.

Because audietur is passive, the thing being heard becomes the grammatical subject. So literally the Latin says the evening song will be heard.

Audietur is the future passive of audire: it will be heard.

Latin often uses the passive where English might prefer an active sentence like they will hear the evening song. The passive keeps the focus on the song, not on the people hearing it. That fits the sentence well, because the important event is the hearing of the song itself.

With in, the ablative usually means location: in / in the atrium.

So in atrio means in the atrium or in the hall/courtyard. If there were motion into the atrium, Latin would normally use in with the accusative instead.

Not really. Latin word order is flexible, and this sentence is arranged in a very natural way:

• Vespere = sets the time first
• postquam omnes convenerint = gives the prior event
• vespertinum carmen in atrio audietur = gives the main action

The basic core is vespertinum carmen audietur. The other parts are placed around it to give context and emphasis. English depends more on word order; Latin depends more on endings.

Here it is nominative singular neuter, agreeing with carmen.

The form vespertinum could also be accusative singular neuter in another sentence, but here carmen is the subject of the passive verb audietur, so both carmen and vespertinum are nominative.

No. Here omnes means all people / everyone.

If Latin wanted to say everything, it would use a neuter form such as omnia. Because convenerint refers to people gathering together, omnes clearly means everyone.