No revisamos el bombín, sino la mirilla.

Revisamos is the first-person plural form of revisar.

It can mean either:

• we inspect / we check → present
• we inspected / we checked → preterite

This form is one of those Spanish verb forms that is spelled the same in both tenses, so context tells you which one is meant.

For example:

• Hoy no revisamos el bombín... = Today we’re not checking the lock cylinder...
• Ayer no revisamos el bombín... = Yesterday we didn’t check the lock cylinder...

In this sentence by itself, learners will often interpret it as present unless context says otherwise.

Because Spanish usually drops subject pronouns when they are not needed.

The ending -amos already tells you the subject is we. So:

• No revisamos... = We don’t inspect/check...

You could add nosotros for emphasis or contrast:

• Nosotros no revisamos el bombín, sino la mirilla.

That might sound like We’re not checking the lock cylinder, but rather the peephole—with extra stress on we.

That is the normal way to make a sentence negative in Spanish:

• no + conjugated verb

So:

• No revisamos = We do not inspect/check

This is much more straightforward than in English, because Spanish does not need an extra helping verb like do.

Here sino introduces a correction or replacement after a negative statement.

The pattern is:

• no X, sino Y
• not X, but rather Y

So the structure means:

• not the lock cylinder, but rather the peephole

It is not just a general contrast. It specifically says that the first thing is rejected and the second thing is the correct alternative.

Because pero and sino are not interchangeable.

Use sino when:

1. the first part is negative, and
2. the second part replaces or corrects the first one.

So:

• No revisamos el bombín, sino la mirilla.
  = We’re not checking the lock cylinder, but rather the peephole.

Use pero for a general contrast, not a correction:

• No revisamos el bombín, pero sí la mirilla.
  = We’re not checking the lock cylinder, but we are checking the peephole.

That version is possible, but it has a slightly different structure and emphasis.

Here it is one word: sino.

That is the conjunction meaning but rather / instead after a negative statement.

Compare:

• No revisamos el bombín, sino la mirilla.
  = not the lock cylinder, but rather the peephole

But si no as two words means if not:

• Si no revisamos la mirilla, habrá problemas.
  = If we don’t inspect the peephole, there will be problems.

So in your sentence, sino must be written as one word.

Because Spanish often omits repeated words when they are already understood.

In this sentence, the verb revisamos is understood in the second part:

• No revisamos el bombín, sino la mirilla.
• literally: We don’t inspect the lock cylinder, but rather the peephole
• understood: We don’t inspect the lock cylinder, but rather [we inspect] the peephole

The second part is just giving the correct direct object.

If you wanted to repeat the verb in a full clause, the natural form would normally be:

• No revisamos el bombín, sino que revisamos la mirilla.

So:

• sino + noun phrase → very natural here
• sino que + clause → if you want to state the second clause fully

Because Spanish often uses the definite article where English might or might not use the, depending on context.

Here the sentence is talking about specific parts being checked, so the articles sound natural:

• el bombín
• la mirilla

Also, every Spanish noun has grammatical gender:

• bombín is masculine → el bombín
• mirilla is feminine → la mirilla

In this context, in Spain:

• bombín usually means the cylinder of a door lock
• mirilla means the peephole in a door

A useful note: bombín can also mean a type of hat in other contexts, but in a sentence about checking parts of a door, it clearly means the lock cylinder.

Because of Spanish stress rules.

Bombín ends in -n. Words ending in a vowel, -n, or -s are normally stressed on the second-to-last syllable. But bombín is stressed on the last syllable:

• bom-BÍN

So it needs a written accent mark.

Mirilla follows the normal stress pattern:

• mi-RI-lla

Since its stress is where Spanish would normally expect it, no accent mark is needed.

Yes, in normal writing it is standard to put a comma before sino in this kind of contrast:

• No revisamos el bombín, sino la mirilla.

That comma helps mark the correction clearly: not X, but rather Y.

In informal writing, punctuation is sometimes inconsistent, but the comma is the standard and recommended choice here.