Quan hi ha molt vent, el pare tanca la finestra.

In this sentence, quan can be understood as when, but in practice it often has the sense of whenever.

Because both clauses are in the present tense, the sentence sounds like a habitual or repeated action:

• Quan hi ha molt vent, el pare tanca la finestra.
• Whenever there is a lot of wind, father closes the window.

So when is correct, but whenever helps show the idea more clearly in English.

Hi ha is the standard Catalan expression for there is or there are.

It comes from the verb haver-hi, which is used to talk about existence:

• Hi ha vent = there is wind
• Hi ha gent = there are people

In this expression, hi is part of the structure. You usually do not translate it word for word on its own here. It is simply necessary in Catalan.

So:

• ha alone does not mean there is
• hi ha does

Yes. Fa molt vent is also very natural Catalan.

There is a small difference in feel:

• fa molt vent = it is very windy
• hi ha molt vent = there is a lot of wind

In many everyday situations, they are very close in meaning. A learner should understand both.

Because vent is a masculine singular noun in Catalan.

So the quantifier must agree with it:

• molt vent = a lot of wind
• molta pluja = a lot of rain
• molts núvols = many clouds
• moltes fulles = many leaves

Also, vent here is being used as an uncountable idea, like wind in English, so the singular makes sense.

Catalan very often uses the definite article with family members when talking about them:

• el pare = father / the father
• la mare = mother
• l’avi = grandfather

This is normal in Catalan, even though English usually does not use the in the same way.

However, when you speak to the person directly, the article is usually dropped:

• Pare, tanca la finestra. = Dad, close the window.

Tanca is the third-person singular present indicative of the verb tancar, meaning to close.

So:

• jo tanco = I close
• tu tanques = you close
• ell/ella tanca = he/she closes

In the sentence, the subject is el pare, so tanca means the father closes.

The present tense is used because the sentence describes a habitual or general situation.

It does not necessarily mean that it is happening right now. Instead, it means this is what usually happens:

• when there is a lot of wind,
• the father closes the window.

This is very similar to English present simple in sentences like:

• When it rains, we stay inside.
• When it is windy, he closes the window.

La finestra uses the definite article because it refers to a specific, understood window.

That is usually the natural choice here: the speaker and listener know which window is meant.

• la finestra = the window
• una finestra = a window

If you said una finestra, it would sound less specific, as if it were just any window.

Yes. You can also say:

El pare tanca la finestra quan hi ha molt vent.

That is completely natural and means the same thing.

The difference is mainly one of emphasis:

• Quan hi ha molt vent, el pare tanca la finestra.
  The windy condition is introduced first.

• El pare tanca la finestra quan hi ha molt vent.
  The main action comes first.

Both are correct.

The comma is used because the sentence begins with a subordinate time clause:

• Quan hi ha molt vent, ...

When this kind of clause comes first in Catalan, a comma is normally used before the main clause.

If the time clause comes at the end, the comma is usually not needed:

• El pare tanca la finestra quan hi ha molt vent.