Turba ante theatrum exspectat; portis apertis, omnes laeti intrant.

Because the grammatical subject is turba, a singular noun meaning crowd. Latin often treats collective nouns as singular in agreement, even though they refer to many individuals. So turba … exspectat = the crowd is waiting (singular verb).

Theatrum is accusative singular because it is the object of the preposition ante (in front of / before), and ante regularly takes the accusative.

In this sentence it’s place: in front of the theater (because people can physically wait there). Ante can be temporal (before in time) or spatial (in front of), and context decides.

That’s an ablative absolute:

• portis = ablative plural of porta (gate)
• apertis = ablative plural perfect passive participle of aperire (opened)

Together it means something like with the gates having been opened / once the gates were opened / when the gates were opened, setting the circumstance for the main action.

Apertis agrees with portis in case, number, and gender:

• portis: ablative plural feminine
• apertis: ablative plural feminine (from apertus, -a, -um)

Agreement is required for participles used adjectivally like this.

No. In an ablative absolute, Latin often omits any form of esse (to be). The participle apertis supplies the idea of a completed action/state (opened), so no separate were is needed in Latin.

The subject is omnes (all [of them]), and it refers back to the people in the crowd. Latin doesn’t need to repeat turba (and in any case turba is singular), so it switches to a plural subject to emphasize the individuals: all (the people) enter.

Because they describe the people who are entering:

• omnes is a nominative plural pronoun/adjective meaning all
• laeti is a nominative plural adjective meaning happy

Both are in the nominative plural because they go with the (understood) plural they of intrant.

It’s not an adverb; it’s an adjective used predicatively: they enter (being) happy / they enter happy. Latin commonly uses an adjective this way instead of an adverb like happily.

Latin word order is flexible. Here omnes laeti is placed together to highlight the subject and its description (all, happy) before the verb. Intrant often comes at the end, but both orders are possible with slightly different emphasis.

• The semicolon links two closely related actions: waiting outside, then entering.
• The comma after apertis marks off the ablative absolute from the main clause, much like setting it off with when/once… in English.

Both are present indicative:

• exspectat = is waiting
• intrant = enter / are entering

Latin present can correspond to English simple present or present progressive depending on context; here it naturally reads like ongoing action: is waiting, then enter as the next event happens.