Revisamos la muestra.

Revisamos is the we form of the verb revisar.

For regular -ar verbs, the nosotros/nosotras ending in the present is -amos:

• yo reviso = I check/review
• tú revisas = you check/review
• él/ella revisa = he/she checks/reviews
• nosotros/nosotras revisamos = we check/review

So Revisamos la muestra uses the verb form that matches we.

Yes. This is a very common question.

With -ar verbs, the nosotros form of the present and the preterite is often identical:

• present: revisamos = we review / we are reviewing / we do review
• preterite: revisamos = we reviewed / we checked

So the form alone does not always tell you whether it is present or past. Context does that.

For example:

• Ahora revisamos la muestra. = now it is clearly present
• Ayer revisamos la muestra. = yesterday it is clearly past

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

• Revisamos la muestra = perfectly natural
• Nosotros revisamos la muestra = also correct, but more explicit

You would include nosotros only if you want emphasis, contrast, or clarity:

• Nosotros revisamos la muestra, pero ellos revisan los datos.

In normal speech, omitting the pronoun is very common.

Because muestra is a feminine singular noun, so it takes the feminine singular definite article la.

• la muestra = the sample
• una muestra = a sample

Spanish uses articles very often, sometimes more often than English does. In this sentence, la simply marks the sample.

Because muestra is a noun with grammatical gender, and its gender is feminine.

That means it goes with:

• la muestra
• una muestra
• esta muestra

It does not mean the sample is biologically female. It is just a grammatical category.

Its plural is:

• la muestra → las muestras

Not usually. This is an important false-friend issue.

In many contexts, Spanish revisar means:

• to check
• to inspect
• to examine
• to review

It often does not mean English to revise in the sense of to rewrite and improve a text.

So in Revisamos la muestra, revisamos is more like we check/examine/review the sample than we revise the sample.

Because la muestra is a direct object, and it refers to a thing, not a person.

Spanish uses the personal a mainly with specific human direct objects:

• Vemos a María.
• Escucho a mi profesor.

But with things, you normally do not use a:

• Revisamos la muestra.
• not Revisamos a la muestra.

So the sentence is correct without a.

A simple English-style guide would be:

reh-bee-SAH-mos lah MWES-trah

A few helpful notes:

• The stress in revisamos is on sa: reviSAmos
• The stress in muestra is on mues: MUEStra
• In most of Spain, v is pronounced very similarly to b
• ue in muestra is pronounced as a diphthong, roughly like we in English well, but not exactly the same

If you want a rough IPA version, it is approximately:

[reβiˈsamos la ˈmwestra]

Yes. That would also be correct, but it means something slightly different in emphasis.

• Revisamos la muestra = simple statement; depending on context, we review/check the sample or we reviewed the sample
• Estamos revisando la muestra = we are reviewing/checking the sample right now, with stronger emphasis on the action being in progress

Spanish often uses the simple present where English might prefer are reviewing, so revisamos can sound very natural in many situations.

You would usually change the noun and article:

• Revisamos la muestra. = we review/check the sample
• Revisamos las muestras. = we review/check the samples

If you wanted a singular indefinite version, you could say:

• Revisamos una muestra. = we review/check a sample