Abrimos la cuenta conjunta con tal de que firmemos.

Con tal de que means provided that, as long as, or on the condition that.

It introduces a condition that must be met. In this sentence, the idea is that opening the joint account depends on our signing.

It is a fairly common structure in Spanish, and it usually sounds a little more formal or explicit than a simple si.

Because con tal de que triggers the subjunctive.

Spanish uses the subjunctive after expressions that introduce:

• conditions
• requirements
• uncertainty
• things that are not yet presented as facts

Here, the signing is a condition, not a stated fact. So Spanish says:

• con tal de que firmemos = provided that we sign

not:

• con tal de que firmamos ❌

Firmemos is the present subjunctive, first person plural of firmar.

So it means we sign in a subjunctive context.

How it is formed:

• infinitive: firmar
• yo form: firmo
• remove -o
• add the subjunctive ending for -ar verbs: -emos

So:

• yo firme
• tú firmes
• él/ella firme
• nosotros firmemos
• ellos firmen

In this sentence, firmemos does not mean let’s sign. Even though the same form can be used for a let’s command in other contexts, here it is clearly a subjunctive verb after con tal de que.

It can be either one, depending on context.

Abrimos can mean:

• we open
• we are opening
• we opened

This happens because for abrir, the nosotros form of the present and the preterite are spelled the same: abrimos.

So only context tells you which one it is.

For example:

• Hoy abrimos la cuenta = Today we open / are opening the account
• Ayer abrimos la cuenta = Yesterday we opened the account

Because Spanish often omits subject pronouns when the verb already makes the subject clear.

Both verbs already show we:

• abrimos = we open / we opened
• firmemos = we sign

So nosotros is not necessary.

You could say Nosotros abrimos la cuenta conjunta..., but that would usually add emphasis, contrast, or clarification.

Because adjectives in Spanish usually agree with the noun in gender and number.

• cuenta is feminine singular
• so the adjective must also be feminine singular
• therefore: conjunta

So:

• cuenta conjunta = joint account

If the noun were masculine, the adjective would change:

• documento conjunto

If it were plural:

• cuentas conjuntas

Yes. Cuenta conjunta is a normal and natural way to say joint account, especially in banking contexts.

You may also hear more specific phrases depending on the country or institution, but cuenta conjunta is widely understandable in Latin American Spanish.

Yes. That is grammatical.

Spanish allows this kind of clause to come either:

• after the main clause: Abrimos la cuenta conjunta con tal de que firmemos
• or before it: Con tal de que firmemos, abrimos la cuenta conjunta

The meaning stays basically the same. The difference is mostly one of emphasis:

• putting con tal de que firmemos first highlights the condition more strongly

It is grammatical and understandable.

That said, depending on context, a speaker might choose a slightly different version:

• Abriremos la cuenta conjunta con tal de que firmemos if they want a clearly future sense
• Abrimos la cuenta conjunta si firmamos los dos in a more everyday style
• Podemos abrir la cuenta conjunta con tal de que firmemos if the idea is we can open it provided we sign

So the original sentence works, but the exact choice depends on whether the speaker wants to sound:

• more formal
• more conversational
• more clearly future-oriented

Yes, when con tal de que introduces a clause with its own verb, that verb normally goes in the subjunctive.

So you get patterns like:

• con tal de que vengas
• con tal de que paguen
• con tal de que firmemos

That is why firmemos is the expected form here.