En verano, el pelo rubio de mi prima parece más claro.

In Spanish, en verano is the normal way to say in summer / during the summer when you are talking generally.

• En verano = in summer, during summer
• En invierno = in winter
• En primavera = in spring
• En otoño = in autumn

You may sometimes hear en el verano, but that usually sounds more specific, like in that particular summer or in certain regional styles. For a general statement, en verano is the most natural choice.

Spanish usually expresses possession with:

• the thing possessed
  • de
    • the owner

So:

• el pelo de mi prima = my cousin’s hair

This is very common in Spanish and often sounds more natural than using a possessive adjective like su.

Compare:

• el pelo de mi prima = my cousin’s hair
• su pelo = her hair

Both are possible, but su pelo can sometimes be ambiguous because su could mean his, her, your, or their depending on context. Using de mi prima makes it completely clear.

Because el pelo is often used as a mass noun in Spanish, similar to hair in English when you mean hair in general.

• el pelo = hair
• un pelo = one hair
• los pelos = hairs

So here, the sentence is talking about your cousin’s hair as a whole, not individual hairs.

They are related, but they do different jobs here.

• rubio means blonde
• claro means light / lighter / pale

So:

• el pelo rubio = blonde hair
• más claro = lighter

The sentence means that your cousin’s blonde hair looks lighter in summer. In other words, the hair is already blonde, and in summer it appears even lighter than usual.

Because rubio describes el pelo, not mi prima.

• prima is feminine, yes
• but pelo is masculine singular

Adjectives agree with the noun they describe:

• el pelo rubio → masculine singular
• la prima rubia → feminine singular

So:

• mi prima rubia = my blonde cousin
• el pelo rubio de mi prima = my cousin’s blonde hair

In your sentence, rubio goes with pelo, so it must be masculine singular.

For the same reason: claro refers to el pelo.

The structure is:

• el pelo ... parece más claro

So the thing that seems lighter is the hair, and pelo is masculine singular. That is why the adjective is claro, not clara.

Because parecer means to seem / to appear, and it matches the idea of how something looks.

• parece más claro = it seems lighter / it looks lighter

Using estar más claro would usually mean it is clearer or it is lighter as a state, but with pelo, parece más claro is more natural when talking about appearance.

Spanish often uses parecer + adjective to describe how something appears:

• Parece cansado. = He looks tired.
• El cielo parece gris. = The sky looks grey.

Yes. Se ve más claro is also natural and means it looks lighter.

Compare:

• parece más claro = seems lighter
• se ve más claro = looks lighter

Both work well here. Parecer can sound a bit more neutral or descriptive, while verse often focuses more directly on visual appearance. In everyday speech, both are common.

Because Spanish often leaves out subject pronouns when the subject is already clear from the sentence.

Here, the subject is:

• el pelo rubio de mi prima

So there is no need to say él.

Spanish does this all the time:

• Mi hermano está cansado. = My brother is tired.
• La casa parece pequeña. = The house seems small.

Adding él here would usually be unnecessary and might sound unnatural unless you were emphasizing the subject.

Because más claro focuses on the degree of lightness, which fits the idea that sunlight can make hair look lighter.

You could say más rubio in some contexts, but it would mean blonder, which is slightly different. In this sentence, más claro is the more natural choice because it describes a change in shade or brightness.

So:

• más claro = lighter
• más rubio = blonder

Both are understandable, but más claro is better for this specific idea.

No. Prima is specifically a female cousin.

• primo = male cousin
• prima = female cousin

So mi prima means my female cousin.

In Spanish, most descriptive adjectives usually come after the noun.

So:

• el pelo rubio = the blonde hair
• la casa blanca = the white house
• un coche rojo = a red car

Sometimes adjectives can come before the noun, but that often changes the tone or meaning. In this sentence, el pelo rubio is the normal, straightforward order.

It is not strictly necessary, but it is very natural.

En verano is an introductory time phrase, and Spanish often puts a comma after this kind of phrase, especially in writing:

• En verano, el pelo rubio de mi prima parece más claro.

You could also write:

• En verano el pelo rubio de mi prima parece más claro.

Both are correct. The comma just helps separate the introductory phrase from the rest of the sentence.