Fue una decisión inteligente.

Here, fue is the third-person singular preterite of ser.

So it means was in the sense of it was or that was.

A useful thing to know: fue can also be a form of ir (went), but not in this sentence. In Fue una decisión inteligente, the context clearly shows it is from ser, because it is linking the subject to a description: una decisión inteligente.

This is one of the most common questions in Spanish.

• fue = preterite
• era = imperfect

In this sentence, fue presents the decision as a completed whole or as a finished judgment about a specific event.

So Fue una decisión inteligente sounds like:

• looking back at one particular decision
• judging it as a complete action

By contrast, Era una decisión inteligente would usually sound more like:

• a description in the past
• background information
• what something seemed to be at the time

Compare:

• Fue una decisión inteligente aceptar el trabajo.
  = Looking back, that choice was smart.

• Era una decisión inteligente en aquel momento.
  = At that time, it was a smart decision.

Spanish often omits subject pronouns when the meaning is clear from context.

So instead of saying:

• Eso fue una decisión inteligente

Spanish very often just says:

• Fue una decisión inteligente

The hidden subject might be something like:

• eso = that
• tomar el tren = taking the train
• aceptarlo = accepting it

Spanish usually does not need to say the subject if it is already understood.

We use ser because the sentence is identifying or classifying something: it says that something was a smart decision.

That is exactly the kind of job ser normally does.

• ser: identity, classification, essential description, evaluation
• estar: state, condition, location

So:

• Fue una decisión inteligente = It was a smart decision.
  This is an evaluation/classification.

You would not normally say Estuvo una decisión inteligente, because estar does not work for this kind of noun phrase.

Because decisión is a countable singular noun, so it normally needs an article here.

• una decisión = a decision

In this sentence, the speaker is saying that something was a smart decision, so Spanish uses the indefinite article just like English does.

Without the article, Fue decisión inteligente, it would sound unnatural in standard Spanish.

Because inteligente is an adjective with the same form for masculine and feminine singular.

So you get:

• un chico inteligente
• una chica inteligente
• un plan inteligente
• una decisión inteligente

The adjective only changes in the plural:

• decisión inteligente
• decisiones inteligentes

So even though decisión is feminine, inteligente stays the same.

The accent mark shows where the stress goes.

• decisión is stressed on the last syllable: de-ci-SIÓN

Without the accent mark, Spanish spelling rules would normally make you stress it differently.

A useful rule:

• words ending in vowel, -n, or -s are normally stressed on the second-to-last syllable
• if the stress is different, Spanish adds a written accent

Since decisión ends in -n but is stressed on the last syllable, it needs the accent: decisión.

In most of Spain, c before i is pronounced like the th in thin.

So decisión is approximately:

• de-thi-SYON

More precisely:

• deci- → de-thi-
• stress on -sión

So in much of Spain:

• decisión ≈ de-thi-SYON

In Latin America and in some parts of Spain, you may hear s instead:

• de-si-SYON

Both are normal depending on accent, but for Spain Spanish, the th sound is what learners are usually taught.

Fue is normally pronounced as one syllable:

• fwe

The u is not fully separate here; it combines with the following vowel sound.

So it is not:

• foo-eh
• foo-ay

It is just:

• fwe

Yes, but the meaning becomes less specific.

• Fue una decisión inteligente = It was a smart decision.
  This clearly talks about a decision.

• Fue inteligente = That was smart / He was smart / She was smart.
  This depends much more on context.

So adding una decisión makes the sentence more precise by naming exactly what was intelligent.