Mi madre mezcla espinacas con champiñones y guisantes cuando quiere preparar algo rápido.

Because the subject is mi madre, which is third person singular: she mixes.

The verb mezclar in the present tense goes like this:

• yo mezclo
• tú mezclas
• él / ella mezcla
• nosotros mezclamos
• vosotros mezcláis
• ellos / ellas mezclan

So mi madre mezcla = my mother mixes.

Spanish often drops subject pronouns because the verb ending usually makes the subject clear.

So instead of saying:

• Mi madre mezcla...
• Ella mezcla...

Spanish normally prefers just:

• Mi madre mezcla...

And later in the sentence, instead of cuando ella quiere, it is very natural to say:

• cuando quiere

because it is still clear that mi madre is the person who wants to prepare something.

The verb mezclar often uses con to show what something is being mixed with.

So:

• mezclar X con Y = to mix X with Y

In this sentence:

• mezcla espinacas con champiñones y guisantes

means she mixes spinach with mushrooms and peas.

This is the most common structure.

In Spanish, when talking about ingredients in a general way, articles are often omitted.

So it is very natural to say:

• mezcla espinacas con champiñones y guisantes

rather than:

• mezcla las espinacas con los champiñones y los guisantes

Using no articles makes it sound like a general cooking statement: these are the ingredients she uses.

If you add the articles, it can sound more like you mean some specific spinach, mushrooms, and peas already known in the context.

Some food words are commonly used in the plural in Spanish, even when English might use a mass noun.

So:

• espinacas = spinach
• guisantes = peas
• champiñones = mushrooms

This is normal Spanish usage. Even though English often says spinach as an uncountable noun, Spanish usually says espinacas.

In Spain, champiñones usually refers specifically to the common cultivated mushrooms, especially button mushrooms.

The broader word for mushrooms is:

• setas = mushrooms

So:

• champiñones = usually the typical supermarket mushrooms
• setas = mushrooms more generally

In this sentence, champiñones sounds very natural for everyday cooking.

Yes, guisantes is the normal word for peas in Spain.

In other Spanish-speaking countries, other words may be used, such as:

• arvejas
• chícharos
• petit pois in some contexts

But for Spanish from Spain, guisantes is exactly the right word to learn.

This is a very common Spanish structure.

After querer (to want), Spanish uses an infinitive:

• quiere preparar = wants to prepare

So the pattern is:

• querer + infinitive

Examples:

• Quiero comer. = I want to eat.
• Quiere descansar. = She wants to rest.
• Queremos salir. = We want to go out.

There is no extra word like English to between them in Spanish.

Because rápido here describes algo (something), not the way she prepares it.

So:

• algo rápido = something quick

This is an adjective phrase.

By contrast:

• rápidamente = quickly
• deprisa = quickly / in a hurry

Those would describe how someone does something, not what kind of thing it is.

Compare:

• quiere preparar algo rápido = she wants to prepare something quick
• quiere preparar algo rápidamente = she wants to prepare something quickly

The first one is much more natural here.

After indefinite words like algo, nada, and alguien, the adjective usually comes after.

So you get:

• algo rápido = something quick
• nada importante = nothing important
• alguien simpático = someone nice

Also, the adjective normally appears in the masculine singular form after algo, so rápido is the expected form.

In Spanish, the present tense is often used for habitual actions, just like in English.

So:

• Mi madre mezcla... cuando quiere...

means something like:

• My mother mixes... when she wants...
• or more naturally in English, My mother mixes... whenever she wants...

It describes what she typically does, not necessarily what she is doing right now.

Yes, but the meaning changes slightly.

• cuando quiere preparar algo rápido = when / whenever she wants to prepare something quick
• si quiere preparar algo rápido = if she wants to prepare something quick

Cuando suggests a repeated or usual situation. Si sounds more conditional.

In this sentence, cuando fits very well because it describes a habitual pattern.

In this context, something quick is the best natural English idea.

It means food that is quick to make or quick to prepare, not food that literally moves fast.

So rápido here refers to convenience and preparation time.

You could say:

• Mi madre mezcla espinacas con champiñones y guisantes cuando ella quiere preparar algo rápido.

This is grammatically correct, but it sounds less natural in normal Spanish because ella is unnecessary.

Spanish usually avoids repeating the subject pronoun unless you want emphasis, contrast, or clarity.

So the original sentence is the more natural version.

Not especially. It is just a natural list of ingredients.

The structure is:

• main ingredient: espinacas
• plus what it is mixed with: champiñones y guisantes

You could change the order of the vegetables and still be grammatical, for example:

• mezcla champiñones con espinacas y guisantes

But the original order may sound more natural if espinacas is the base of the dish.