Quan el temps canvia molt, la meva mare té mal de cap.

Yes, el temps can mean both time and weather in Catalan. In this sentence, it means weather.

So:

• el temps passa = time passes
• el temps canvia = the weather changes

This is very common in Catalan, and the intended meaning usually becomes clear from the context.

Quan means when.

It introduces a time clause:

• Quan el temps canvia molt = when the weather changes a lot

This works much like English when in sentences such as When the weather changes, my mother gets a headache.

In normal Catalan word order, molt usually comes after the verb when it modifies how much something happens.

So:

• canvia molt = changes a lot

This is the natural order in Catalan. Putting molt before the verb would usually sound unnatural here.

The comma separates the opening time clause from the main clause.

• Quan el temps canvia molt, = when the weather changes a lot,
• la meva mare té mal de cap. = my mother has a headache.

This is similar to English, where a comma is often used after an introductory clause.

Catalan normally uses the definite article with possessives in many everyday expressions:

• la meva mare = my mother
• el meu pare = my father
• la meva germana = my sister

This is one of the big differences from English. In English, we say my mother without the, but in Catalan the article is usually included.

Catalan often uses tenir = to have for physical conditions.

So:

• té mal de cap literally = has pain of head
• natural English meaning = has a headache / gets a headache

This is a fixed and very common expression. Similar patterns include:

• tenir gana = to be hungry
• tenir set = to be thirsty
• tenir son = to be sleepy

Mal de cap is the standard expression for headache.

Literally:

• mal = pain / ache / badness
• de = of
• cap = head

So mal de cap literally means something like pain of the head.

You will see the same pattern in other body-related expressions:

• mal de coll = sore throat / neck pain, depending on context
• mal d’esquena = back pain
• mal de panxa = stomach ache

The accent distinguishes té from te.

• té = he/she/it has
• te = tea

So in this sentence:

• la meva mare té mal de cap = my mother has a headache

The accent is important because it changes the meaning.

Yes, té is the present tense of tenir and it agrees with la meva mare.

The subject is singular, third person:

• jo tinc = I have
• tu tens = you have
• ell/ella té = he/she has

Since la meva mare means my mother, the correct form is té.

Yes, depending on context, English may use either singular or plural:

• my mother has a headache
• my mother gets headaches

The Catalan sentence itself uses the singular expression té mal de cap, which often refers to the condition in a general way. In natural English, both versions can work, depending on what nuance you want.

It most naturally sounds like a general or habitual statement.

So the idea is:

• whenever the weather changes a lot, this tends to happen

Catalan present tense is often used for general truths, habits, and repeated situations, just like English present tense can be.

Yes, Quan canvia molt el temps is also possible.

Both mean the same thing:

• Quan el temps canvia molt
• Quan canvia molt el temps

The first version is a very straightforward, neutral word order. The second is also natural, but may feel slightly more stylistic or emphatic depending on context.

Here cap means head.

It is pronounced roughly like kahp, with a short a sound, not like English cap.

Be careful: cap can also have other meanings in Catalan depending on context, such as none/any in some structures. But in mal de cap, it clearly means head.