A praia fica mais bonita quando o dia está ensolarado.

In this sentence, ficar means something like to become, to get, or to look in a certain condition.

So A praia fica mais bonita... suggests that the beach becomes or looks prettier when it is sunny.

If you said A praia é mais bonita..., it would sound more like a general comparison or a more permanent quality. Fica highlights a change depending on the situation.

Because bonita describes praia, and praia is a feminine noun in Portuguese.

Adjectives usually agree with the noun they describe in:

• gender: masculine or feminine
• number: singular or plural

So:

• a praia bonita
• o lugar bonito
• as praias bonitas
• os lugares bonitos

Here, praia is feminine singular, so bonita must also be feminine singular.

Yes. A very common way to make comparatives in Portuguese is:

mais + adjective

So:

• mais bonita = prettier / more beautiful
• mais caro = more expensive
• mais interessante = more interesting

In this sentence, mais bonita means prettier.

You can compare it to English:

• pretty → prettier
• Portuguese usually does not change the adjective itself; it just adds mais

So Portuguese says:

• bonita → mais bonita

Because estar is normally used for a state or condition, especially one that can change.

A sunny day is a condition, so Portuguese says:

• O dia está ensolarado.

Using ser would usually describe something as a more permanent or defining characteristic.

For example:

• Brasília é ensolarada. = Brasília is sunny in general.
• Hoje o dia está ensolarado. = Today the day is sunny.

So in your sentence, está ensolarado fits because it describes the day’s current weather.

Because ensolarado describes dia, not praia.

The structure is:

• o dia está ensolarado

Since dia is masculine singular, the adjective must also be masculine singular:

• dia ensolarado

If the noun were feminine, the adjective would change:

• a manhã está ensolarada
• a tarde está ensolarada

So the sentence has two different adjective agreements:

• bonita agrees with praia
• ensolarado agrees with dia

It can mean both, depending on context.

In this sentence, because it expresses a general truth or repeated situation, quando is often closer to whenever:

• A praia fica mais bonita quando o dia está ensolarado.
• The beach looks prettier whenever the day is sunny.

English often uses when in the same way, so when is still a perfectly good translation.

This is a very common learner question.

Portuguese uses:

• present indicative after quando for general truths, habits, or repeated situations
• future subjunctive after quando for a specific future situation

So here:

• quando o dia está ensolarado = when/whenever the day is sunny
  This is a general statement.

But if you were talking about a future occasion:

• Quando o dia estiver ensolarado, vamos à praia.
  = When the day is sunny / When it turns out to be sunny, we’ll go to the beach.

So está is correct because the sentence is about a general pattern, not one future event.

Portuguese uses definite articles much more often than English does.

So:

• a praia
• o dia

sound completely natural here.

Even when English might say just beach in a general sense, Portuguese often prefers a praia.

Leaving the articles out would usually sound unnatural in normal speech:

• Praia fica mais bonita... sounds incomplete or telegraphic, like a headline.

So for a normal full sentence, A praia and o dia are the expected forms.

Yes, it is natural. Portuguese often talks about the day to describe the weather.

So o dia está ensolarado means something like:

• the day is sunny
• it’s a sunny day
• it’s sunny out

Other natural alternatives are:

• Está ensolarado.
• Faz sol.
• O tempo está ensolarado.

All of these can work, but o dia está ensolarado is perfectly normal.

Yes. A very natural alternative is:

Quando o dia está ensolarado, a praia fica mais bonita.

This has the same meaning. The difference is mainly focus and style:

• A praia fica mais bonita... starts with the main idea
• Quando o dia está ensolarado... starts with the condition

When the quando clause comes first, Portuguese usually uses a comma:

• Quando o dia está ensolarado, a praia fica mais bonita.

When it comes second, a comma is usually not needed:

• A praia fica mais bonita quando o dia está ensolarado.