Falta una retención, de ahí que revisemos el cálculo.

Here falta comes from faltar, which often means to be missing or to be lacking.

So:

• Falta una retención = A withholding/deduction is missing / There is a missing withholding item

It is singular because the thing that is missing is una retención, which is singular. In Spanish, the verb agrees with the thing that is missing:

• Falta una retención = one item is missing
• Faltan dos retenciones = two items are missing

This is a very common pattern in Spanish.

Because faltar is the natural verb when something is absent from a list, document, calculation, file, etc.

Spanish often uses faltar where English uses expressions like:

• to be missing
• to be absent
• to be lacking

So Falta una retención sounds natural for A withholding item is missing.
Estamos perdiendo... would usually mean we are losing..., which is a different idea.

Retención has several meanings depending on context. In financial or administrative language, it often means a withholding, deduction, or amount held back.

In Spain, a learner will often see retención in contexts like:

• tax withholding
• payroll deductions
• amounts retained in an invoice or calculation

So in this sentence, it most likely refers to some withholding/deduction that should be included in the calculation but is not.

De ahí que is a connector meaning something like:

• hence
• therefore
• which is why
• that is why

So:

• Falta una retención, de ahí que revisemos el cálculo.
• A withholding item is missing; hence / that is why we are checking the calculation.

It links a cause to a consequence in a fairly formal or written style.

Because de ahí que normally triggers the subjunctive.

So after de ahí que, Spanish usually uses:

• revisemos = present subjunctive

not:

• revisamos = present indicative

This is one of the key grammar points in the sentence.

Compare:

• Falta una retención, de ahí que revisemos el cálculo.
• There is a missing withholding item, which is why we are checking the calculation.

The subjunctive appears because de ahí que presents the second clause as a consequence derived from the first.

In standard Spanish, yes, normally it does.

Typical pattern:

• de ahí que + subjunctive

Examples:

• De ahí que sea importante.
• De ahí que lo hagamos así.
• De ahí que revisemos el cálculo.

For a learner, it is best to treat de ahí que as a fixed trigger for the subjunctive.

Yes, and that would be more straightforward and conversational.

Compare:

• Falta una retención, por eso revisamos el cálculo.
• Falta una retención, de ahí que revisemos el cálculo.

Difference:

• por eso = more common, direct, everyday
• de ahí que = more formal, more written, and followed by subjunctive

So both can express a similar idea, but de ahí que sounds more polished or formal.

Because Spanish usually drops subject pronouns when the verb already makes the subject clear.

• revisemos already tells you the subject is we
• so nosotros is unnecessary unless you want emphasis or contrast

You could say:

• ..., de ahí que nosotros revisemos el cálculo.

But that would sound more emphatic, as if you were contrasting we with someone else.

Spanish uses the definite article much more often than English.

So el cálculo means:

• the calculation

In context, it refers to a specific calculation already known in the situation. That is why the definite article sounds natural.

Spanish would usually not say just revisemos cálculo here. The article is needed.

Yes, it is natural and expected here because de ahí que introduces a consequence related to the first clause.

The comma helps separate:

1. the statement of the problem: Falta una retención
2. the consequence: de ahí que revisemos el cálculo

So the punctuation supports the logic of the sentence.

It sounds fairly formal, especially because of de ahí que.

Parts of the sentence:

• Falta una retención = neutral and common
• de ahí que revisemos el cálculo = more formal, written, or professional

This kind of sentence would fit well in:

• reports
• emails
• accounting or administrative communication
• formal explanations

In everyday speech, many people would prefer something simpler like:

• Falta una retención, por eso revisamos el cálculo.

Literally, de ahí means from there, but you should not translate the whole expression word for word.

As a fixed expression, de ahí que means something like:

• hence
• therefore
• which is why

So it is much better to learn it as one unit rather than trying to build the meaning from each word separately.