No verão, eu gosto de comer melancia fria.

Because no is the contraction of em + o.

• em = in
• o = the
• em o verão becomes no verão

This kind of contraction is very common in Portuguese:

• em + a = na
• em + os = nos
• em + as = nas

So no verão means in the summer.

Because verão is a masculine noun in Portuguese, so it takes o, not a.

• o verão = the summer
• no verão = in the summer

If the noun were feminine, you would use na instead.

You can absolutely omit eu.

Portuguese often leaves out subject pronouns when the verb already makes the subject clear. Since gosto clearly means I like, eu is not necessary.

So both are correct:

• No verão, eu gosto de comer melancia fria.
• No verão, gosto de comer melancia fria.

Including eu can add a little emphasis or clarity, but in everyday Portuguese it is often omitted.

Because the verb gostar normally requires the preposition de.

So you say:

• gosto de comer = I like to eat
• gosto de melancia = I like watermelon

This is something English speakers often have to get used to, because English like does not need a preposition here, but Portuguese gostar does.

Yes, you could, and it would sound natural.

The difference is small:

• gosto de comer melancia fria = I like eating cold watermelon
• gosto de melancia fria = I like cold watermelon

The version with comer focuses a bit more on the action of eating. The version without comer is more direct and often more natural in casual speech.

Because here melancia is being used in a general sense, not referring to a specific watermelon.

So:

• comer melancia = to eat watermelon / to eat some watermelon in general
• comer a melancia = to eat the watermelon, a specific one

In this sentence, the speaker means watermelon as a food in general, so no article is needed.

Because fria agrees with melancia, and melancia is feminine.

• melancia = feminine
• fria = feminine form of frio

This is adjective agreement:

• melancia fria = cold watermelon
• sumo frio = cold juice

The adjective has to match the noun in gender and number.

In Portuguese, adjectives often come after the noun.

So:

• melancia fria = cold watermelon

That is the most normal order here.
Sometimes adjectives can come before the noun, but that often changes the tone, emphasis, or sounds less natural. In this sentence, melancia fria is the standard and expected word order.

Here it means cold, especially in the sense of chilled.

With food, frio/fria usually means that something is cold in temperature:

• água fria = cold water
• melancia fria = cold/chilled watermelon

So this suggests watermelon that has been cooled, probably in the fridge.

It is optional, but it is perfectly natural.

No verão is an introductory time phrase: In summer. In Portuguese, a comma is often used after this kind of phrase, especially in writing, to make the sentence easier to read.

So both are acceptable:

• No verão, eu gosto de comer melancia fria.
• No verão eu gosto de comer melancia fria.

The version with the comma is a little clearer on the page.

The main thing to notice is the -ão ending, which is nasal.

verão is pronounced approximately like vuh-RAHWN with a nasal ending, not a full English own sound.

A few useful points for European Portuguese:

• the e in verão is often reduced, sounding more like a soft uh
• the stress is on the last syllable: ve-RÃO
• ão is nasal, which English does not really have

That nasal ending is very common in Portuguese, so it is worth practicing carefully.

The sentence works in both European and Brazilian Portuguese.

Grammar and vocabulary here are standard in both varieties:

• No verão
• gosto de comer
• melancia fria

The biggest difference would be pronunciation, and in Brazil people may be slightly more likely to keep eu in some contexts. But as a sentence, it is completely natural in Portugal and also understandable and correct in Brazil.