Dimecres, un actor i una cantant assagen sobre l'escenari abans que arribi el públic.

In Catalan, days of the week can often be used without a preposition when giving the time of an event.

So:

• Dimecres = Wednesday / On Wednesday
• Dimecres, ... sets the time frame first

The comma is not absolutely required in every context, but here it helps separate the time expression from the rest of the sentence. It works a bit like an English introductory phrase:

• On Wednesday, an actor and a singer rehearse...

So Dimecres is functioning as a time marker placed at the front for emphasis or clarity.

These are the indefinite articles:

• un = a before a masculine singular noun
• una = a before a feminine singular noun

So:

• un actor = an actor (masculine)
• una cantant = a singer (feminine)

A useful detail: cantant has the same form for masculine and feminine in many contexts. The article tells you the gender:

• un cantant = a male singer
• una cantant = a female singer

So even though cantant does not change form here, the article shows that the singer is female.

Assagen means they rehearse.

Its infinitive is assajar, meaning to rehearse.

This form is:

• present tense
• 3rd person plural

So:

• un actor i una cantant assagen = an actor and a singer rehearse

A few related forms:

• jo assajo = I rehearse
• tu assages = you rehearse
• ell/ella assaja = he/she rehearses
• nosaltres assagem = we rehearse
• vosaltres assageu = you all rehearse
• ells/elles assagen = they rehearse

Because the subject has two people:

• un actor i una cantant = an actor and a singer

When two singular nouns are joined by i (and), they form a plural subject, so the verb must also be plural.

That is why Catalan uses:

• assagen = they rehearse

not the singular assaja.

Sobre l'escenari means on the stage.

Breakdown:

• sobre = on / over / upon
• l'escenari = the stage

In Catalan, a l'escenari is also very common for on stage / at the stage area, especially in everyday speech.
Sobre l'escenari can sound slightly more descriptive or literary, focusing on being physically on the stage surface or space.

So in practice:

• assagen a l'escenari
• assagen sobre l'escenari

can both make sense, though the exact nuance may vary a bit by context and style.

Catalan often contracts the article el before a vowel.

So:

• el escenari becomes l'escenari

This is called elision.

Other examples:

• l'actor = the actor
• l'obra = the play
• l'escola = the school

So l'escenari simply means the stage.

Because abans que (before) normally triggers the subjunctive in Catalan when the action has not happened yet.

In this sentence:

• abans que arribi el públic = before the audience arrives

The audience has not arrived yet at the moment of rehearsal, so Catalan uses the subjunctive:

• arribi = present subjunctive of arribar

This is a very important pattern:

• abans que + subjunctive

Examples:

• Marxa abans que comenci la pluja.
  Leave before the rain starts.

• Ho faré abans que sigui massa tard.
  I’ll do it before it’s too late.

So arribi is not random; it is required by the structure.

Arribi is the present subjunctive, 3rd person singular, of arribar (to arrive).

Here is the present subjunctive of arribar:

• jo arribi
• tu arribis
• ell/ella arribi
• nosaltres arribem
• vosaltres arribeu
• ells/elles arribin

In the sentence, the subject is el públic, which is singular, so Catalan uses:

• arribi

Because públic is a collective noun. It refers to a group of people, but grammatically it is treated as singular.

So Catalan says:

• el públic arriba
• el públic aplaudeix
• el públic està content

not plural verb forms.

This is similar to English the audience, which is also often treated as singular depending on the variety of English.

So in the sentence:

• abans que arribi el públic

the verb is singular because el públic is singular.

Yes, cantant can be more than one thing in Catalan.

It can be:

1. a noun: singer
2. a gerund-like form / present participle-type form from cantar: singing

In this sentence, it is clearly a noun because it comes after an article:

• una cantant = a singer

So here it cannot mean singing. The article una makes its role clear.

Catalan is a pro-drop language, which means subject pronouns are often omitted because the verb ending already gives enough information.

However, in this sentence the subject is not a pronoun like they. It is a full noun phrase:

• un actor i una cantant

That phrase is included because it gives the actual information about who is rehearsing.

If the people were already known, Catalan could later say simply:

• Assagen sobre l'escenari... = They rehearse on the stage...

So Catalan can omit subject pronouns, but not when the speaker wants to identify the people involved.

The given order is natural, but Catalan has some flexibility.

The sentence as written is:

• Dimecres, un actor i una cantant assagen sobre l'escenari abans que arribi el públic.

This is very clear and neutral:

• time first
• subject
• verb
• place
• subordinate clause

Other orders are possible for emphasis, for example:

• Un actor i una cantant assagen sobre l'escenari dimecres, abans que arribi el públic.
• Sobre l'escenari, un actor i una cantant assagen abans que arribi el públic.

These alternatives may shift emphasis slightly, but the original sentence is perfectly normal.

A rough pronunciation guide is:

• assagen ≈ uh-SAH-jen / əˈsa.ʒən in Central Catalan

A few helpful points:

• ss gives a voiceless s sound
• the g before e sounds like the soft sound in French j or the s in measure
• the final -en is usually unstressed

This is only an approximation for English speakers, but it helps distinguish it from forms with a hard g sound, which this does not have.

Abans que is a conjunction meaning before.

It introduces a subordinate clause:

• abans que arribi el públic = before the audience arrives

It is best learned as a fixed expression:

• abans de + infinitive when the subject stays the same
• abans que + subjunctive clause when there is a full clause with its own subject

Compare:

• Assagen abans d'entrar a escena.
  They rehearse before going on stage.

• Assagen abans que arribi el públic.
  They rehearse before the audience arrives.

So the sentence uses abans que because there is a new subject in the second clause: el públic.