Cuando hace calor, mi padre abre la ventana y yo bajo la persiana.

Here it most naturally means when/whenever in a general, habitual sense.

Because the sentence uses the present tense, Cuando hace calor... often means something like:

Whenever it’s hot, my father opens the window and I lower the blind.

So it is not necessarily talking about one specific moment; it can describe a regular pattern.

Hace calor is the normal Spanish weather expression for it’s hot.

Spanish often uses hacer + weather noun, for example:

• hace calor = it’s hot
• hace frío = it’s cold
• hace viento = it’s windy

Está caliente usually means it is hot/warm to the touch, for an object, food, drink, etc., not the weather in general.

So for weather, hace calor is the natural choice.

That is just one of the fixed uses of hacer in Spanish.

Although hacer often means to do/make, in weather expressions it works differently. In hace calor, it does not literally mean makes heat in normal English terms; it is simply the standard Spanish structure.

You can think of hace calor as a whole chunk to learn, rather than translating each word separately.

They are all in the present indicative:

• hace = 3rd person singular of hacer
• abre = 3rd person singular of abrir
• bajo = 1st person singular of bajar

In this sentence, the present tense describes a usual action or routine.

In modern Spanish, a possessive like mi, tu, su, nuestro, etc. normally goes directly before the noun, with no article:

• mi padre = my father
• mi casa = my house

So el mi padre is not standard Spanish here.

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

So mi padre abre is completely natural without adding él.

The pronoun yo appears here for contrast or emphasis:

• mi padre opens the window
• I lower the blind

Including yo helps highlight the difference between what the father does and what the speaker does.

Here bajo is a verb: the first-person singular present of bajar.

So it means I lower / I pull down.

It is true that bajo can also be:

• an adjective: low
• a preposition/adverb in some contexts: under
• a noun/adjective in music: bass

But in this sentence, because it is followed by la persiana, it is clearly the verb bajar.

In Spain, la persiana usually means a blind or roller shutter, often the kind you can raise and lower over a window.

So bajar la persiana means to lower/pull down the blind or shutter.

In Spain this is a very common household item, and it is especially associated with blocking sunlight and heat.

There is also an idiomatic use of bajar la persiana meaning to close a shop/business, but in this sentence it is clearly the literal meaning because of la ventana and the hot weather.

Because they refer to specific, known objects in the situation: the window and the blind/shutter in that room or house.

Spanish uses the definite article very naturally in cases like this, especially when the object is understood from context.

English would also often say the window and the blind here, so this part matches English fairly well.

Not in this sentence.

With the present tense on both sides, it usually describes a general habit or repeated situation:

When/whenever it’s hot, this is what we do.

If Spanish wanted to talk about a future event after cuando, it would often still use the present in the cuando clause, but the rest of the sentence would make the future meaning clear. Here, though, the whole sentence feels habitual.

Because Cuando hace calor is an introductory subordinate clause placed before the main clause.

In Spanish, it is very common to put a comma after that kind of opening clause:

Cuando hace calor, mi padre abre la ventana...

If the cuando clause came later, the comma would often be omitted:

Mi padre abre la ventana y yo bajo la persiana cuando hace calor.

Yes. That is a very natural translation.

Bajar literally means to lower, but in everyday English, pull down the blind may sound more idiomatic depending on the type of blind.

So possible translations include:

• I lower the blind
• I pull down the blind
• I lower the shutter (if it is more like a shutter)

Yo can definitely be omitted:

• Cuando hace calor, mi padre abre la ventana y bajo la persiana.

That still clearly means my father opens the window and I lower the blind, because bajo already shows I.

But mi padre cannot be omitted unless the context already makes it clear who abre refers to. If you removed it with no context, the sentence would become ambiguous.

So in this sentence:

• omitting yo is easy
• omitting mi padre depends on context