Esa idea fue estúpida.

Because idea is a feminine noun in Spanish, so the demonstrative has to match it.

• esa = feminine singular
• ese = masculine singular

So:

• esa idea = that idea
• ese libro = that book

Even though idea starts with an i, it is still feminine: la idea, una idea, esa idea.

Esa means that for a feminine singular noun.

Spanish has a three-way distinction that English often does not make clearly:

• esta = this
• esa = that
• aquella = that over there / that one farther away

So esa idea is that idea, usually something not right next to the speaker, or something already mentioned in the conversation.

Because esa already does the job of introducing the noun.

In Spanish, demonstratives like este, esta, ese, esa, aquel, aquella normally replace the article rather than appearing with it.

So you say:

• esa idea
• not la esa idea

Compare:

• la idea = the idea
• esa idea = that idea

This is one of the most common questions in Spanish past tenses.

• fue is the preterite of ser
• era is the imperfect of ser

In Esa idea fue estúpida, fue presents the idea as a completed thing and gives a judgment about it as a whole: That idea was stupid.

Using era would sound more like background description or an ongoing characteristic:

• La idea era estúpida = The idea was stupid / The idea used to be stupid / descriptive background

In many contexts, both can appear, but they do not feel exactly the same.

A simple way to think of it:

• fue = a finished event or whole situation
• era = description, background, ongoing state

So if someone proposed something and you are judging that specific proposal afterward, fue sounds very natural.

Because adjectives usually have to agree in gender and number with the noun they describe.

Since idea is feminine singular, the adjective must also be feminine singular:

• idea estúpida
• not idea estúpido

More examples:

• un plan estúpido
• una idea estúpida
• unos comentarios estúpidos
• unas decisiones estúpidas

The accent mark shows where the stress goes.

So estúpida is stressed on tú:

• es-TÚ-pi-da

Without the accent mark, a Spanish speaker would expect a different stress pattern.

Accent marks in Spanish are very important because they often tell you how a word is pronounced.

In standard Spanish, fue is pronounced roughly like fweh, as one syllable.

A few useful notes:

• f sounds like English f
• ue forms a single syllable here
• the u is pronounced, unlike in words such as guerra or queso

So:

• fue ≈ fweh

In Spain, the pronunciation is essentially the same, though accent details vary by region.

It is pronounced roughly:

• es-TOO-pee-da

More carefully in Spanish:

• es-TÚ-pi-da

A few details:

• e is a clear Spanish e, not like the English sound in see
• u is like oo in food
• i is like ee in see
• d between vowels in Spanish is usually softer than English d

Yes. Fue can come from either ser or ir in the preterite:

• ser → fue = was
• ir → fue = went

You know which one it is from the context.

In Esa idea fue estúpida, fue must mean was, because that idea went stupid does not make sense. So here it is clearly from ser.

This overlap is completely normal in Spanish.

It can definitely sound harsh.

Calling an idea estúpida is a strong negative judgment, similar to stupid in English. Depending on tone and context, it can sound blunt or rude.

Softer alternatives include:

• mala = bad
• poco inteligente = not very smart
• desacertada = misguided / unfortunate
• una mala idea = a bad idea

So Esa idea fue estúpida is understandable and natural, but it is not especially polite.

Yes, but the meaning or emphasis may shift slightly.

The basic sentence is:

• Esa idea fue estúpida.

Other possible versions:

• Fue una idea estúpida. = It was a stupid idea.
• Esa fue una idea estúpida. = That was a stupid idea.

These are all natural, but they are not identical in focus:

• Esa idea fue estúpida emphasizes that idea
• Fue una idea estúpida presents the judgment more generally
• Esa fue una idea estúpida can sound a bit more contrastive or emphatic

In Spanish, most descriptive adjectives normally come after the noun.

So:

• una idea estúpida
• un coche rojo
• una película interesante

Putting an adjective before the noun is sometimes possible, but it often changes the tone, emphasis, or style. With estúpida, the normal position is after the noun.

Yes, you could, but it gives a slightly different feeling.

• Esa idea fue estúpida = a completed judgment about a specific idea or action
• Esa idea era estúpida = more descriptive, background-like, or reflective

For many learners, the safest first interpretation is:

• fue = that was stupid
• era = that was stupid in a more descriptive or ongoing sense

So both are possible in the right context, but fue works very well if you are evaluating a finished action or suggestion.