Quando corro ao sol, fico com a testa cheia de suor.

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

• corro = I run
• fico = I get / I become / I end up

Because -o clearly shows 1st person singular, eu is not necessary.

So:

• Quando corro ao sol... = When I run in the sun...
• Quando eu corro ao sol... is also possible, but eu would usually be added only for emphasis or contrast.

Here quando means when / whenever, describing a habitual or repeated situation.

So the present tense is natural:

• Quando corro ao sol, fico... = When/Whenever I run in the sun, I get...

It is not necessarily talking about one specific future event. It is describing what generally happens.

This is very common in Portuguese:

• Quando estudo, desligo o telemóvel.
• Quando chove, fico em casa.

Corro is the 1st person singular present of correr = to run.

So:

• eu corro = I run

In this sentence, it can mean:

• when I run
• when I go running

It does not specifically mean I am running right now. It is the normal tense for a general statement.

Ao is the contraction of:

• a + o = ao

Here, ao sol means something like:

• in the sun
• under the sun
• exposed to the sun

In Portuguese, ao sol is the natural expression for being in direct sunlight.

Examples:

• ficar ao sol = to stay in the sun
• trabalhar ao sol = to work in the sun
• correr ao sol = to run in the sun

For this kind of meaning, ao sol is the usual idiomatic choice.

• ao sol focuses on exposure to sunlight
• no sol is much less idiomatic in this context

So Portuguese prefers:

• estar ao sol
• sentar-se ao sol
• correr ao sol

An English speaker may want to translate in the sun literally, but in Portuguese the most natural expression is often ao sol.

Ficar com is a very common Portuguese structure. Here it means something like:

• to end up with
• to get
• to be left with

So:

• fico com a testa cheia de suor = I end up with my forehead covered in sweat

This is more natural in Portuguese than translating word-for-word from English.

Compare:

• Fico cansado. = I get tired.
• Fico com fome. = I get hungry.
• Fico com a camisa molhada. = My shirt ends up wet / I end up with a wet shirt.

Portuguese very often uses the definite article with body parts where English uses a possessive.

So instead of saying my forehead, Portuguese naturally says the forehead, because it is already obvious whose forehead it is.

• a testa = literally the forehead
• but in context it means my forehead

This is very common:

• Lavei as mãos. = I washed my hands.
• Doe-me a cabeça. = My head hurts.
• Parti o braço. = I broke my arm.

Using a minha testa is possible, but it is less natural here unless you want emphasis.

Cheia agrees with testa, which is a feminine singular noun.

• a testa → feminine singular
• therefore: cheia

Agreement in Portuguese works like this:

• o copo cheio = the full glass
• a garrafa cheia = the full bottle
• os copos cheios
• as garrafas cheias

So in this sentence:

• a testa cheia de suor = the forehead full of sweat

Cheio de / cheia de means full of.

So:

• cheia de suor = literally full of sweat

In natural English, you would usually say:

• covered in sweat
• sweaty
• dripping with sweat

But the Portuguese expression is completely normal.

The structure is:

• cheio / cheia + de + noun

Examples:

• cheio de água = full of water
• cheia de gente = full of people
• cheia de suor = full of sweat / covered in sweat

Yes. A sentence like fico com a testa suada is possible and natural.

The difference is mainly one of style and nuance:

• a testa suada = a sweaty forehead
• a testa cheia de suor = a forehead full of sweat, which sounds a bit more vivid and visual

So cheia de suor emphasizes the visible presence of sweat a little more strongly.

Both are correct; the version in your sentence is just more expressive.

Yes. Portuguese can also say:

• Fico com a testa cheia de suor quando corro ao sol.

This means the same thing.

The version with Quando... at the beginning is very natural because it sets up the situation first:

• Quando corro ao sol, fico com a testa cheia de suor.

Also, the comma is normal when the quando clause comes first.

Yes. Testa is the normal everyday word for forehead.

Another word you may see is fronte, but that is usually more:

• formal
• literary
• elevated in style

So in everyday speech, testa is the best choice here.

• a testa cheia de suor sounds natural and conversational.