Quando me aproximo da janela, sinto a brisa fresca da manhã.

Portuguese often drops the subject pronoun when the verb ending already makes the subject clear.

• aproximo = I approach / I get close
• sinto = I feel

So eu is not necessary. You could say Quando eu me aproximo da janela, eu sinto..., but it is less natural unless you want emphasis or contrast.

Here quando means when in the sense of whenever or when(ever this happens).

The present tense is normal because the sentence describes:

• a general truth
• a habitual action
• or a vivid present description

So Quando me aproximo da janela... means something like Whenever I go near the window... or When I go near the window...

If you wanted a future meaning, Portuguese often uses the future subjunctive:

• Quando me aproximar da janela, ... = When I approach / when I come near the window (in the future), ...

The verb here is aproximar-se de, which means to approach, to move closer to, or to get close to.

So:

• aproximar-se de algo = to approach something
• aproximo-me da janela / me aproximo da janela = I approach the window / I move closer to the window

The me is a reflexive/pronominal pronoun that belongs to the verb in this use.

Without me, the verb usually changes meaning or structure:

• aproximar algo de algo = to bring something closer to something

Example:

• Aproximo a cadeira da mesa. = I move the chair closer to the table.

So in your sentence, me is necessary.

Yes, but in this sentence me aproximo is the correct and natural choice because quando attracts the pronoun before the verb.

In European Portuguese, pronoun placement is very important:

• after certain words like quando, não, que, se, etc., the pronoun usually goes before the verb
• this is called proclisis

So:

• Quando me aproximo da janela... = correct
• Quando aproximo-me da janela... = not standard

If the clause stood alone, European Portuguese would often prefer:

• Aproximo-me da janela.

Da is a contraction of:

• de
  • a = da

The verb pattern is aproximar-se de:

• aproximar-se de + noun

Since janela is feminine and usually takes the definite article a, you get:

• de a janela → da janela

So aproximo-me da janela literally means I approach of the window, but naturally in English it is just I approach the window / I move closer to the window.

It can be understood either way, depending on context.

Most naturally, it suggests a habitual or repeated experience:

• Whenever I go near the window, I feel the cool morning breeze.

But Portuguese also uses the present tense for a vivid immediate description, especially in narrative or literary style:

• When I approach the window, I feel the cool morning breeze.

So the grammar allows both. Context tells you which one is meant.

Because Quando me aproximo da janela is an introductory subordinate clause.

The comma separates:

• the when-clause: Quando me aproximo da janela
• from the main clause: sinto a brisa fresca da manhã

This is very similar to English:

• When I approach the window, I feel the cool morning breeze.

Here sinto means I feel in a physical sensory way.

So:

• sinto a brisa = I feel the breeze

The verb sentir is quite broad and can mean:

• to feel physically
• to feel emotionally
• sometimes to sense / notice
• in some contexts, even to smell

But in this sentence, because of a brisa fresca, the natural meaning is clearly to feel the breeze on your skin.

In Portuguese, adjectives usually come after the noun, so:

• brisa fresca = fresh/cool breeze

Also, adjectives must agree with the noun in gender and number.

• brisa is feminine singular
• so the adjective must also be feminine singular
• fresco → fresca

Compare:

• vento fresco = cool wind
• brisa fresca = cool breeze

Putting the adjective before the noun is sometimes possible, but it often changes the tone or sounds more literary or marked. Brisa fresca is the normal order here.

Here fresca means something like cool, fresh, or pleasantly cool.

It does not usually mean strongly cold in this sentence. A brisa fresca da manhã suggests a gentle, refreshing morning breeze.

So the best idea is:

• fresh breeze
• cool breeze

rather than something like freezing breeze.

This is a very important difference.

• da manhã means of the morning and describes the noun
• de manhã means in the morning and describes when something happens

So:

• a brisa fresca da manhã = the cool morning breeze / the cool breeze of the morning
• De manhã, sinto a brisa fresca. = In the morning, I feel the cool breeze.

In your sentence, da manhã belongs to brisa. It tells you what kind of breeze it is.

In European Portuguese, many unstressed vowels are reduced, so it may sound more compact than you expect from the spelling.

A rough guide is:

Quando me aproximo da janela, sinto a brisa fresca da manhã.

Approximate European Portuguese sound: KWAN-doo m'uh-pruh-ZEE-moo d'ah zhuh-NEL-luh, SIN-too uh BREE-zuh FRESH-kuh d'uh muhn-YAH

A few useful points:

• j in janela sounds like the s in measure
• x in aproximo sounds like z here
• unstressed e and o are often weakened in European Portuguese
• nh in manhã sounds like ny in canyon

This is only an approximation, but it helps explain why European Portuguese can sound more reduced and less syllable-by-syllable than Brazilian Portuguese.