Em julho, eu compro um uniforme leve para o estágio de verão.

With month names, Portuguese normally uses em + month: em julho, em agosto, em dezembro.
So em julho is the natural way to say in July.

No julho is not idiomatic. If you want a longer version, you can say no mês de julho.

Because Em julho is an introductory time phrase placed at the start of the sentence. The comma helps separate that setting from the main clause.

It is often used, but it is not always mandatory.
You could also write Em julho eu compro um uniforme leve... and it would still be correct.

That is true: Portuguese often leaves out subject pronouns when the verb already shows who is doing the action. So Compro um uniforme leve... is perfectly grammatical.

Including eu can add:

• emphasis
• contrast
• clarity

For example, eu compro may sound like I buy it, especially if someone else does something different.

Compro is the 1st person singular present indicative of comprar.

In Portuguese, the present tense can be used for:

• habits or repeated actions
• general statements
• sometimes planned or scheduled future actions, depending on context

So this sentence can sound like a routine statement, or in context it may point to something arranged for July. If you want a clearer future meaning, Portuguese could also use vou comprar or comprarei.

Yes. Uniforme is a masculine noun in Portuguese, so it takes um and o:

• um uniforme
• o uniforme

This is a good reminder that noun gender in Portuguese does not depend only on the final letter. Even though uniforme ends in -e, it is masculine.

In Portuguese, adjectives often come after the noun, especially when they describe a straightforward quality.

So:

• um uniforme leve = a light/lightweight uniform

Putting the adjective before the noun is sometimes possible in Portuguese, but it often sounds more literary, emphatic, or changes the nuance. Here, uniforme leve is the normal order.

Because leve is one of many adjectives that have the same singular form for masculine and feminine nouns.

So you get:

• um uniforme leve
• uma camisa leve

Only the plural changes:

• uniformes leves
• camisas leves

Because para expresses purpose or intended use here: the uniform is for the internship.

So para o estágio means something like:

• for the internship
• intended for the internship

Por o estágio would not fit this meaning. Por usually means things like by, through, because of, or for the sake of, depending on context.

The article o suggests a specific internship, one that is known from context.

Portuguese uses definite articles more often than English does, so where English might say for summer internship or for the internship, Portuguese often naturally says para o estágio.

If you remove the article, para estágio de verão, it sounds more generic and less tied to one particular internship.

De verão modifies estágio, not uniforme.

So:

• o estágio de verão = the summer internship

This is a very common Portuguese pattern: de + noun is often used where English uses a noun as an adjective or a compound noun.

Other examples:

• curso de verão = summer course
• roupa de inverno = winter clothes

The tricky part is lh, which is a special Portuguese sound.

A rough European Portuguese approximation of julho is ZHOO-lyoo, with the stress on the first syllable. The lh is similar to the lli sound in some pronunciations of million, though it is not exactly the same.

So the key thing is:

• ju = like zhoo
• lh = a soft ly sound

The accent in estágio shows where the stress goes: es-TÁ-gio.

It is there because the word does not follow the default stress pattern that its spelling would otherwise suggest. Accent marks in Portuguese are very important because they help you know:

• which syllable is stressed
• sometimes what vowel quality to use

In European Portuguese, the beginning often sounds reduced, so estágio may sound roughly like ish-TA-zhyoo.