A ideia é excelente.

A is the feminine singular definite article, so it means the.
Because ideia is a feminine noun, it takes a in the singular.

So:

• a ideia = the idea
• uma ideia = an idea / one idea

Portuguese uses articles more often than English, so keeping a here sounds natural.

In Portuguese, every noun has a grammatical gender, either masculine or feminine. Ideia is simply a feminine noun, so it goes with:

• a ideia
• uma ideia
• adjectives and other words that match feminine nouns when needed

This is grammatical gender, not biological gender.

Both ser and estar can mean to be, but they are used differently.

Here, é is from ser, which is used for:

• definitions
• general statements
• judgments or classifications
• more permanent or inherent qualities

So A ideia é excelente sounds like a general evaluation of the idea.

Está would sound more temporary or situational. In some contexts, A ideia está excelente could be possible, but it would usually suggest something like the idea is looking excellent right now rather than a basic judgment about what it is.

The accent matters a lot here.

• é = is
• e = and

So the accent distinguishes two different words.

It also shows the stressed vowel sound in é.

Modern Brazilian Portuguese spells it ideia, without an accent.

Older Brazilian spelling often used idéia, but that changed with spelling reform. So today:

• correct modern spelling: ideia
• older spelling: idéia

If you see idéia in older materials, that is why.

A rough Brazilian pronunciation is:

a ee-DEH-ya eh eh-seh-LEN-chee

More approximately:

• a = short ah
• ideia = ee-DEH-ya
• é = eh
• excelente = eh-seh-LEN-chee

A more IPA-like version is:

/a iˈdɛjɐ ɛ esɛˈlẽtʃi/

The exact sound will vary a bit by region.

Because excelente is a predicate adjective here. It describes a ideia through the verb ser:

• A ideia é excelente = subject + verb + adjective

This is different from putting the adjective directly before the noun:

• uma excelente ideia = an excellent idea

Both are correct, but the structure is different.

Not in the singular. Excelente has the same form for masculine and feminine singular:

• o plano é excelente
• a ideia é excelente

In the plural, it becomes:

• os planos são excelentes
• as ideias são excelentes

So it changes for number, but not for gender in the singular.

Yes, but the meaning changes.

• A ideia é excelente = the idea is excellent
• Uma ideia é excelente = an idea is excellent / one idea is excellent

The second one is less natural unless you are contrasting it with others or speaking more generally. In most everyday situations, if you are talking about a specific idea, A ideia is the natural choice.

Normally, no. In standard Portuguese, you would usually need the article or another determiner:

• A ideia é excelente
• Essa ideia é excelente
• Uma ideia é excelente

Saying Ideia é excelente by itself sounds incomplete or unnatural in normal speech.

You can, but it is marked and emphatic. It sounds more literary, dramatic, or contrastive.

The normal, neutral order is:

• A ideia é excelente

Use Excelente é a ideia only if you want special emphasis or a stylistic effect.

Yes. That would mean that idea or this idea, depending on context.

Compare:

• A ideia é excelente = the idea is excellent
• Essa ideia é excelente = that/this idea is excellent

Using essa makes the reference more specific, especially if you are pointing to a particular idea in conversation.