A l'estiu, m'agrada llegir al pati perquè hi ha ombra.

Yes. A l'estiu means in the summer or in summer.

Catalan often uses a + article with time expressions:

• a l'estiu = in the summer
• a l'hivern = in the winter
• al matí = in the morning

Here, l' is the article el shortened before a vowel:

• a + el estiu → a l'estiu

So A l'estiu is a very natural way to say in summer.

Because agradar works differently from English to like.

In Catalan, agradar literally means something like to be pleasing to. So:

• m'agrada llegir = reading is pleasing to me
• literally: to me, reading pleases

That is why:

• m' = to me
• agrada = pleases / is pleasing

The thing that is pleasing here is llegir al pati, which is treated as a singular idea, so the verb is singular: agrada.

Compare:

• M'agrada llegir. = I like reading.
• M'agraden els llibres. = I like books.

M' is the shortened form of em, which means to me.

Before a vowel, em usually becomes m':

• em agrada → m'agrada

So:

• m'agrada = it pleases me / I like

Other similar forms are:

• t'agrada = you like
• li agrada = he/she likes
• ens agrada = we like

No—this is exactly what Catalan normally does after agradar when talking about activities.

• M'agrada llegir. = I like reading / I like to read.
• M'agrada cuinar. = I like cooking / I like to cook.
• M'agrada caminar. = I like walking / I like to walk.

So Catalan uses the infinitive where English often uses either:

• reading, or
• to read

Al is the contraction of a + el.

• a + el → al

So:

• al pati = in the patio / in the courtyard / in the yard

This is very common in Catalan:

• al cotxe = in/to the car
• al parc = in/to the park
• al matí = in the morning

Also, Catalan often uses a where English might use in, at, or to, depending on context.

Not always exactly.

Pati can mean:

• patio
• courtyard
• yard
• sometimes even a school playground, depending on context

In this sentence, al pati probably means a patio, courtyard, or yard area outside the house or building.

This is a very common question.

• perquè = because
• per què = why / for what reason

In your sentence:

• perquè hi ha ombra = because there is shade

Examples:

• Vaig entrar perquè plovia. = I went inside because it was raining.
• Per què plores? = Why are you crying?

So here it is one word, perquè, because it gives the reason.

Hi ha means there is or there are.

It is the standard Catalan way to say that something exists or is present somewhere.

Examples:

• Hi ha un llibre a la taula. = There is a book on the table.
• Hi ha moltes persones aquí. = There are many people here.

In your sentence:

• hi ha ombra = there is shade

It is best to learn hi ha as a fixed expression.

Because ombra here means shade, not a shadow.

When Catalan talks about something uncountable or general, it often uses no article:

• hi ha ombra = there is shade
• hi ha aigua = there is water
• hi ha menjar = there is food

If you said una ombra, that would sound more like a shadow:

• Hi ha una ombra a la paret. = There is a shadow on the wall.

So in this sentence, ombra means the general condition of being shaded.

It can mean either, depending on context.

• ombra = shadow
• ombra = shade

In this sentence, it clearly means shade:

• M'agrada llegir al pati perquè hi ha ombra. = I like reading in the patio because there is shade.

That is, the place is shaded and therefore comfortable.

The comma separates the introductory time phrase from the rest of the sentence.

• A l'estiu, m'agrada llegir...

This is similar to English:

• In summer, I like reading...

The comma helps readability, especially when the sentence begins with a time expression. In some cases, people may omit it, but using it here is very natural.

No. Hi ha is the correct expression.

You need hi ha to mean there is / there are.
Without hi, the sentence would be ungrammatical.

So:

• perquè hi ha ombra = correct
• perquè ha ombra = incorrect

The given order is the most natural one, but Catalan does allow some flexibility.

The original sentence:

• A l'estiu, m'agrada llegir al pati perquè hi ha ombra.

A few variations are possible, depending on emphasis:

• M'agrada llegir al pati a l'estiu perquè hi ha ombra.
• Perquè hi ha ombra, a l'estiu m'agrada llegir al pati.

But the original version sounds very natural and clear:

1. time: A l'estiu
2. preference: m'agrada
3. activity: llegir al pati
4. reason: perquè hi ha ombra