Fue una idea brillante.

Both come from ser (to be), but they present the idea differently:

• fue = preterite: it presents the idea as a completed fact or a specific event/judgment.
• era = imperfect: it sounds more like background description, an ongoing quality, or a general characterization.

So:

• Fue una idea brillante. = It was a brilliant idea.
  This sounds like a definite assessment, often about one particular moment or action.
• Era una idea brillante. = It was a brilliant idea.
  This can sound more descriptive, like you are setting the scene or talking about how the idea seemed over a period of time.

In many contexts both are possible, but fue is the more natural choice when you are simply giving a completed evaluation.

Fue is from the verb ser (to be).

It is the third person singular preterite form, so it can mean:

• he/she/it was
• you were (formal usted)

In this sentence, fue refers to una idea, so it means it was.

Una is the feminine singular indefinite article, meaning a or an.

It is used because idea is:

• singular
• feminine

So:

• un libro = a book
• una idea = an idea

Even though idea ends in -a, what matters is that it is grammatically feminine, so una is correct.

In Spanish, nouns have grammatical gender. Idea is a feminine noun, so it takes feminine articles and adjectives where needed.

That is why you say:

• una idea
• la idea

This is something you generally just learn with the noun. A useful habit is to memorize nouns together with their article:

• la idea
• el problema
• la mano

That helps because endings do not always guarantee gender.

In Spanish, adjectives usually come after the noun.

So the normal order is:

• una idea brillante

rather than putting the adjective first.

This is the most neutral and common position. In some cases, adjectives can go before the noun for emphasis or style, but that is less common and can slightly change the tone.

So for a straightforward sentence, una idea brillante is the expected order.

Some Spanish adjectives have the same form for masculine and feminine.

Brillante is one of them.

So you get:

• un plan brillante
• una idea brillante

It does change for number, though:

• una idea brillante
• unas ideas brillantes

So the adjective agrees in number, but not in gender in this case.

Yes, you can, but it sounds a bit more literary, emphatic, or stylistically marked.

Compare:

• Fue una idea brillante. = neutral, standard, most common
• Fue una brillante idea. = more emphasis on brilliant

In everyday Spanish, especially for learners, una idea brillante is the safest and most natural choice.

Yes. Brillante can mean:

• brilliant / clever / excellent
• shiny / sparkling

The meaning depends on context.

Examples:

• una idea brillante = a brilliant idea
• un metal brillante = a shiny metal

In your sentence, because it describes idea, it clearly means brilliant, not physically shiny.

Yes. Spanish often leaves out subject pronouns when they are clear from the verb or the context.

So instead of saying:

• Eso fue una idea brillante
• La propuesta fue una idea brillante
  (depending on what you mean)

Spanish can simply say:

• Fue una idea brillante.

The subject is understood from the situation. This is very normal in Spanish.

Yes, fue is also a preterite form of ir (to go), which can be confusing.

So fue can mean either:

• was from ser
• went from ir

You tell the difference from the context.

Here, Fue una idea brillante can only be from ser, because went a brilliant idea makes no sense.

A simple pronunciation guide is:

fweh OO-na ee-DEH-a bree-YAN-te

A few key points:

• fue is one syllable in normal speech: fweh
• una = OO-na
• idea usually has three syllables: i-de-a
• ll in standard Spain Spanish is usually pronounced like y: bri-YAN-te

Also, in natural speech, fue una often links together smoothly.

No. Neither idea nor fue takes a written accent mark here.

• idea has no accent mark because it follows normal stress rules.
• fue is written without an accent in modern standard Spanish.

So the correct spelling is:

Fue una idea brillante.

Yes. It is a very natural sentence for praising a specific idea, suggestion, or decision.

For example, after someone proposes something smart, you could say:

• Fue una idea brillante.

Depending on context, Spanish speakers might also say things like:

• Fue una gran idea.
• Qué idea tan brillante.
• Esa fue una idea brillante.

Your sentence is perfectly natural and positive.

The plural version would be:

Fueron unas ideas brillantes.

Changes:

• fue → fueron because the subject is plural
• una → unas
• idea → ideas
• brillante → brillantes

So all the elements agree in number.