Aceptamos a condición de que nos avisen antes.

Because a condición de que triggers the subjunctive. This expression introduces a condition, requirement, or something that has not yet happened, so Spanish uses the subjunctive after it.

• a condición de que nos avisen = on condition that they inform us / let us know
• That idea is not a fact yet; it is a condition that must be met.

So:

• avisan would sound wrong here
• avisen is the expected form

It means on the condition that, provided that, or as long as.

It is a fixed expression used to set a requirement:

• Aceptamos a condición de que... = We agree/accept on the condition that...

It is fairly formal or neutral in tone. In everyday speech, people might also use expressions like:

• siempre que
• con tal de que
• siempre y cuando

These can be similar, though the tone and exact nuance may change slightly.

It can mean either, depending on context.

The verb aceptar literally means to accept, but in many contexts it is naturally translated as:

• we agree
• we accept
• we’re willing to do it

So in this sentence, Aceptamos could mean something like:

• We agree, provided that...
• We accept, on condition that...

The most natural English translation often depends on what is being accepted.

Nos means us.

In que nos avisen, it is the indirect object pronoun:

• avisar a alguien = to inform someone / let someone know
• nos avisen = that they inform us / let us know

So the structure is:

• avisar
  • a alguien
• avisarnos / nos avisen = inform us

Avisen is the third person plural present subjunctive form of avisar, so it usually means they inform / they let know in a subjunctive context.

So que nos avisen means:

• that they let us know
• that they inform us

Depending on the wider context, they could refer to a specific group already mentioned.

It can also sometimes be a formal you plural form in some varieties of Spanish, but in Spain that would normally be understood as they unless the context clearly indicates otherwise.

Because this is not a simple statement of fact. The phrase a condición de que requires the subjunctive, and the subjunctive form for ellos/ellas/ustedes is avisen.

Compare:

• Nos avisan antes. = They let us know beforehand.
  This is a statement, so it uses the indicative.

• Aceptamos a condición de que nos avisen antes. = We agree on condition that they let us know beforehand.
  This is a condition, so it uses the subjunctive.

Because antes can be used on its own when the reference is understood.

Here, antes means beforehand / in advance.

• nos avisen antes = they let us know beforehand

You use antes de when what comes next is stated explicitly:

• antes de salir = before leaving
• antes de la reunión = before the meeting

But if no specific event is named, antes by itself is fine.

It is followed by que + a conjugated verb in the subjunctive.

So:

• a condición de que nos avisen ✓

Not:

• a condición de avisar ✗ in this same meaning

This expression specifically uses que plus a finite verb.

Yes, con la condición de que is also possible and means something very similar: with the condition that / on the condition that.

However, a condición de que is a very common set phrase and sounds very natural here.

So both can work, but:

• a condición de que = very standard fixed expression
• con la condición de que = also correct, sometimes a bit more explicit or formal

It sounds neutral to somewhat formal.

The phrase a condición de que is more structured and formal than very casual everyday speech. In conversation, someone might more naturally say:

• Aceptamos si nos avisan antes.
• Vale, pero avisadnos antes.
• De acuerdo, siempre que nos avisen antes.

Your original sentence is perfectly correct, but it has a slightly more formal or written tone.

Yes. Avisar can mean:

• to inform
• to let someone know
• to warn

The exact meaning depends on context.

In this sentence, nos avisen antes most naturally means:

• they let us know beforehand
• they inform us in advance

It does not strongly sound like a warning here.

Because Spanish often leaves subject pronouns out when the verb ending already shows who the subject is.

• avisen already tells you it is they (or you plural in some contexts)
• So ellos is unnecessary unless you want emphasis or contrast

For example:

• Aceptamos a condición de que nos avisen antes. = normal
• Aceptamos a condición de que ellos nos avisen antes. = emphasizes they, perhaps in contrast with someone else

Aceptamos is the present indicative, first person plural of aceptar.

It can mean:

• we accept
• we are accepting
• sometimes even we agree

Like many Spanish present-tense forms, its exact English translation depends on context.

So the sentence combines:

• Aceptamos = present indicative
• avisen = present subjunctive

That mix is very normal in Spanish when a main clause is followed by a conditional or hypothetical subordinate clause.

Yes. Very often, Spanish uses the present to talk about future arrangements, decisions, or conditions.

So Aceptamos a condición de que nos avisen antes can refer to a present decision about a future action:

• We’ll agree, provided that they let us know in advance

This is completely natural in Spanish. The present tense here does not mean the action is happening right now; it can express a current stance about what must happen later.