Mi primo termina de aplastar el aguacate y luego lo aliña con limón y sal.

Yes. Primo is a male cousin, and prima is a female cousin.

So:

• mi primo = my male cousin
• mi prima = my female cousin

Unlike English cousin, Spanish usually makes that distinction.

Because after terminar, Spanish normally uses de + infinitive to say to finish doing something.

So:

• termina de aplastar = finishes mashing/crushing
• termina de comer = finishes eating
• termina de estudiar = finishes studying

Without de, it would not sound correct here.

A useful comparison:

• terminar de + infinitive = to finish doing something
• acabar de + infinitive = to have just done something

So in this sentence, termina de aplastar means he completes that action.

Yes. Aplastar literally means to crush, to squash, or to flatten, and it can work for food.

With avocado, a native speaker might also say:

• machacar el aguacate
• triturar el aguacate in some contexts

But aplastar el aguacate is understandable and natural enough, especially if the idea is physically pressing it down with a fork or similar tool.

Yes, aguacate is a masculine noun, so it takes:

• el aguacate
• un aguacate

Its gender is something you simply learn with the word. The ending -e does not automatically tell you whether a noun is masculine or feminine, so you need to memorize it.

Because it is masculine singular, the object pronoun later is lo.

Lo refers back to el aguacate.

So:

• el aguacate = the avocado
• lo = it

Spanish often mentions the noun first and then replaces it with a pronoun to avoid repetition.

It comes before aliña because with a conjugated verb, object pronouns normally go before the verb:

• lo aliña
• la come
• los prepara

You would attach the pronoun to the end only in certain cases, such as with an infinitive, a gerund, or an affirmative command.

In Spain, aliñar commonly means to season or to dress food by adding things like oil, vinegar, lemon, salt, and similar ingredients.

Here, lo aliña con limón y sal means he seasons/dresses it with lemon and salt.

This verb is especially common in Spain. In other Spanish-speaking regions, people may more often use verbs like aderezar or simply another expression depending on the country.

Because Spanish often leaves out the article when talking about ingredients or substances in a general way.

So con limón y sal means with lemon and salt as ingredients, not with the lemon and the salt as specific, already identified items.

This is very natural in Spanish:

• con azúcar
• con sal
• con aceite
• con limón

If you added articles, it would sound more specific:

• con el limón y la sal = with the specific lemon and salt already being referred to

Yes, in many cases luego and después can both mean then or afterwards.

So these are both natural:

• y luego lo aliña
• y después lo aliña

In this sentence, luego works well as a sequence word, showing the next step in a process.

In Spain, luego is very common in this kind of context.

Because el aguacate is a thing, not a person.

The personal a is usually used before a specific human direct object, and sometimes pets or personified beings. For example:

• Veo a mi primo
• Llamo a María

But with things, you normally do not use it:

• aplasta el aguacate
• compra el pan
• abre la puerta

So no a is needed here.

Because Spanish often uses the simple present to describe steps in a process, a routine, or a sequence of actions.

That makes it very natural in contexts like:

• recipes
• instructions
• demonstrations
• narrating what someone typically does

So termina... y luego lo aliña... can sound like a description of what he does next.

English sometimes prefers the progressive in similar situations, but Spanish uses the simple present much more broadly.

The key part is the letter ñ, which is different from plain n.

Aliña is pronounced roughly like ah-LEE-nya.

A few points:

• a as in father
• li like lee
• ñ sounds like the ny in canyon or senior

So it is not a-lee-na, but a-lee-nya.