A ideia é brilhante.

It breaks down like this:

• A = the (feminine singular definite article)
• ideia = idea
• é = is (from the verb ser)
• brilhante = brilliant / bright / shiny, depending on context

So the structure is:

article + noun + verb + adjective

Very close to English: The idea is brilliant.

Because a means the, while uma means an / one.

• A ideia é brilhante. = The idea is brilliant.
• Uma ideia é brilhante. = An idea is brilliant.

In most contexts, a ideia refers to a specific idea already known in the conversation. That is why a is natural here.

The accent in é is important because it distinguishes the verb é (is) from e (and).

• é = is
• e = and

As for ideia, the current standard spelling in Portuguese is ideia, with no accent. Older spelling used idéia, but that is no longer the standard after spelling reforms.

So modern Brazilian Portuguese writes:

• ideia
• assembleia
• jiboia

without the old accent.

A rough Brazilian Portuguese pronunciation would be:

ah ee-DEI-a eh bree-LYAN-chee

A few notes:

• ideia sounds roughly like ee-DEI-a
• é sounds like eh
• brilhante has lh, which is not a normal English l sound; it is closer to the lli sound in million for many learners
• the stress in brilhante is on -lhan-

You do not need a perfect English-style approximation, but it helps to know where the stress goes.

Portuguese often uses ser for qualities seen as essential, defining, or general judgments.

• A ideia é brilhante. = the idea is brilliant as a judgment about what it is

Using estar would usually suggest a temporary state or a condition at the moment.

• A ideia está brilhante. sounds unusual in most normal situations

So for saying an idea is clever, excellent, or brilliant, ser is the natural choice.

Not in the singular.

Brilhante is one of many adjectives that have the same form for masculine and feminine singular:

• o plano é brilhante
• a ideia é brilhante

The plural does change:

• os planos são brilhantes
• as ideias são brilhantes

So:

• singular: brilhante
• plural: brilhantes

It can mean both.

Literal meaning:

• shiny
• bright
• sparkling

Figurative meaning:

• brilliant
• excellent
• clever

In A ideia é brilhante, the meaning is clearly figurative: brilliant.

Yes, but that changes the structure and the feel.

• A ideia é brilhante. = The idea is brilliant.
• A brilhante ideia = The brilliant idea

The original sentence is a complete statement.
A brilhante ideia is just a noun phrase unless you add more words.

Also, adjective position can affect style. Putting the adjective before the noun can sound more literary, emphatic, or stylized in Portuguese.

Yes.

• A ideia é brilhante. = The idea is brilliant.
• Essa ideia é brilhante. = That idea is brilliant. / This idea is brilliant. depending on context

Essa points to the idea more directly. It is very common in spoken Brazilian Portuguese when you want to identify which idea you mean.

Usually no, not in normal everyday Portuguese.

A native speaker would normally say:

• A ideia é brilhante.
• Uma ideia brilhante...
• Essa ideia é brilhante.

Without the article, Ideia é brilhante sounds incomplete, unnatural, or like a headline or very telegraphic style.

This is something English speakers often need to watch out for, because Portuguese uses articles more often than English.

Here are the most useful variations:

Negative

• A ideia não é brilhante. = The idea is not brilliant.

Question

• A ideia é brilhante? = Is the idea brilliant?

Plural

• As ideias são brilhantes. = The ideas are brilliant.

Notice the changes:

• a → as
• é → são
• brilhante → brilhantes