La declaración conjunta cambió, de ahí que revisemos la declaración individual.

Because de ahí que normally triggers the subjunctive.

Here, revisemos is the present subjunctive of revisar in the nosotros form. After de ahí que, Spanish presents the second clause as a result, conclusion, or consequence of the first one, and that structure usually takes the subjunctive:

• La declaración conjunta cambió, de ahí que revisemos la declaración individual.

If you used revisamos, it would sound wrong in this structure.

De ahí que is a fixed expression meaning something like:

• hence
• therefore
• that’s why
• which is why

It connects the first idea to a consequence or conclusion:

• La declaración conjunta cambió → the joint return changed
• de ahí que revisemos la declaración individual → which is why we review the individual return

It is more formal than simply saying por eso.

In standard usage, yes, normally it is.

Examples:

• Hubo errores, de ahí que repitamos el proceso.
• El plazo cambió, de ahí que tengamos que esperar.

So if you see de ahí que, you should strongly expect a subjunctive verb after it.

Revisemos is:

• the verb revisar
• present subjunctive
• first person plural (nosotros = we)

So:

• yo revise
• tú revises
• él/ella/usted revise
• nosotros revisemos
• vosotros reviséis
• ellos/ellas/ustedes revisen

In this sentence, the subject is not stated, but it is understood as we.

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

So instead of saying:

• ... de ahí que nosotros revisemos...

Spanish usually prefers:

• ... de ahí que revisemos...

The -emos ending already tells you it means we.

Cambió is the pretérito indefinido (simple past) of cambiar. It presents the change as a completed event:

• cambió = changed

That fits well here because the sentence first states a completed fact, and then gives the consequence:

• The joint return changed, which is why we are reviewing the individual return.

If the speaker wanted to describe an ongoing or background situation, another tense might be used, but cambió is the natural choice for a completed change.

In Spain, declaración conjunta and declaración individual are very common tax-related terms.

• declaración conjunta = joint tax return
• declaración individual = individual tax return

Even though declaración can mean declaration in a general sense, in this context it very often refers to a tax return.

Spanish often repeats the noun when it helps keep the sentence clear and precise.

So instead of saying something like:

• La declaración conjunta cambió, de ahí que la revisemos.

the sentence says:

• La declaración conjunta cambió, de ahí que revisemos la declaración individual.

This makes the contrast between joint and individual very clear.

Because Spanish uses definite articles very naturally with specific nouns.

Here, both are understood as specific things:

• la declaración conjunta
• la declaración individual

In English, article use is often reduced in certain contexts, but in Spanish this is completely normal and expected.

Sometimes yes, but the grammar changes.

With por eso, you normally use the indicative, not the subjunctive:

• La declaración conjunta cambió, por eso revisamos la declaración individual.

With de ahí que, you use the subjunctive:

• La declaración conjunta cambió, de ahí que revisemos la declaración individual.

So the meaning is similar, but the structure is different. De ahí que sounds a bit more formal or written.

The comma marks a pause between the first statement and the consequence that follows.

Structure:

• La declaración conjunta cambió,
• de ahí que revisemos la declaración individual.

This punctuation is very common when de ahí que introduces the result of the previous clause.

Yes, but it means something different.

• de ahí by itself usually means from there or from that
• de ahí que is a fixed connector meaning hence / which is why

Compare:

• De ahí salimos temprano. = We left from there early.
• Cambió el plan, de ahí que esperemos. = The plan changed, which is why we’re waiting.

So in your sentence, de ahí que should be learned as one expression.

It can feel slightly flexible in English, depending on context.

In this structure, revisemos mainly expresses the consequence of the earlier fact. So possible English renderings include:

• which is why we review...
• which is why we are reviewing...
• which is why we should review...

The exact choice depends on the wider context. The key grammar point in Spanish is not tense in the English sense, but the fact that de ahí que requires the subjunctive.