En casa ponemos aceite y vinagre en la ensalada, pero mi hermano prefiere mayonesa.

Here en casa means at home or in our household.

It is a very common fixed expression in Spanish:

• En casa comemos tarde. = At home we eat late.
• Me quedo en casa. = I’m staying at home.

Spanish usually says en casa without an article, not en la casa, when it means at home in a general sense.

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

• ponemos = we put / we add
• so nosotros is not necessary

You could say Nosotros ponemos... if you want extra emphasis or contrast, but in normal speech it is usually omitted.

Literally, yes: poner means to put. But in this sentence it is used in a more natural food-related sense, closer to:

• we put
• we add
• we use

So ponemos aceite y vinagre en la ensalada is naturally understood as we put/add oil and vinegar on the salad.

In English, put is possible, but add may sound more natural in some contexts.

With food, Spanish often uses poner X en Y to mean put/add X in/on Y.

So:

• poner aceite en la ensalada = to put oil on the salad

Using a is less natural here. En is the usual choice for adding an ingredient or dressing to something.

A learner-friendly way to think of it:

• en = the ingredient goes into/on the salad as part of it

Good question. This difference is very common in Spanish.

la ensalada

Here it refers to a specific salad, probably the one being served or discussed:

• en la ensalada = in/on the salad

mayonesa

Here it is used in a general sense, like a substance or ingredient:

• prefiere mayonesa = he prefers mayonnaise

Spanish often leaves out the article with general, uncountable foods after verbs like preferir, comer, beber, etc.

Compare:

• Prefiero café. = I prefer coffee.
• Ella come arroz. = She eats rice.

But you could use the article if you mean a specific mayonnaise:

• prefiere la mayonesa casera = he prefers the homemade mayonnaise

Because aceite and vinagre are being used in a general, ingredient-like sense.

So Spanish commonly says:

• ponemos aceite y vinagre not necessarily
• ponemos el aceite y el vinagre

Without the articles, it sounds like we use oil and vinegar in general.

If you added the articles, it would sound more specific:

• ponemos el aceite y el vinagre = perhaps referring to particular oil and vinegar already known in the situation

Yes. Preferir is a stem-changing verb.

In the present tense, the e in the stem changes to ie in most forms:

• yo prefiero
• tú prefieres
• él/ella prefiere
• nosotros preferimos
• vosotros preferís
• ellos prefieren

So mi hermano prefiere mayonesa means my brother prefers mayonnaise.

Notice that nosotros and vosotros do not have the stem change.

Spanish often uses a possessive like mi, tu, su, etc. when identifying a family member in a straightforward way.

• mi hermano = my brother

You could sometimes omit it if the context is already very clear, but mi hermano is the normal, natural choice here.

Here pero is correct because the sentence is making a contrast:

• At home we put oil and vinegar on salad, but my brother prefers mayonnaise.

Sino is used after a negative statement to correct or replace something:

• No prefiere aceite y vinagre, sino mayonesa. = He doesn’t prefer oil and vinegar, but rather mayonnaise.

So in your sentence, pero is the right conjunction.

Very often, yes.

In Spain, when people simply say aceite in a food context, it is often understood to mean olive oil, especially in everyday meals like salad.

If someone wants to be specific, they can say:

• aceite de oliva = olive oil
• aceite de girasol = sunflower oil

But in many homes in Spain, aceite on its own commonly suggests olive oil.

Yes. Echar is also very common in Spain for adding food, dressing, salt, sugar, and so on.

So both are natural:

• Ponemos aceite y vinagre en la ensalada
• Echamos aceite y vinagre en la ensalada

Very roughly:

• poner = to put
• echar = to add / pour / throw in

In everyday speech, both can work here.

Both are in the present indicative.

• ponemos = we put / we add
• prefiere = he prefers

In this sentence, the present is used for a habitual action or a general preference:

• At home, we put oil and vinegar on salad
• my brother prefers mayonnaise

So it is not necessarily about one single moment; it can describe what is normally done.

Yes, Spanish word order is flexible, but the original order is very natural:

• En casa ponemos aceite y vinagre en la ensalada, pero mi hermano prefiere mayonesa.

It starts with En casa to set the context: at home / in our house.

You could rearrange parts for emphasis, for example:

• Ponemos aceite y vinagre en la ensalada en casa... but that sounds less natural
• Mi hermano, sin embargo, prefiere mayonesa. adds stronger contrast

So the original sentence is a very normal, neutral way to say it.

Yes. In Spain, you may also see or hear mahonesa.

Both forms exist:

• mayonesa
• mahonesa

Today, mayonesa is very common and widely accepted, so the sentence sounds perfectly normal.