Fue un error terrible.

Fue is the third-person singular preterite form of both ser and ir.

So fue can mean:

• was / were from ser
• went from ir

You know which one it is from the context.

In Fue un error terrible, the words after fue are un error terrible, which identify or classify something. That makes fue a form of ser, so it means was.

If it were ir, you would normally expect a destination or movement idea, for example:

• Fue al médico = he/she went to the doctor

This is one of the most common questions in Spanish.

• fue = preterite
• era = imperfect

In this sentence, fue presents the mistake as a completed fact or judgment. It sounds like the speaker is evaluating a specific event: It was a terrible mistake.

Era un error terrible is also possible, but it usually feels more like:

• background description
• an ongoing situation in the past
• a less bounded, more descriptive statement

Very roughly:

• Fue un error terrible = a specific mistake, seen as a finished whole
• Era un error terrible = it was a terrible mistake, with more descriptive/background flavor

Because the sentence is identifying/classifying something as un error terrible.

Spanish usually uses ser for:

• identity
• classification
• definitions
• essential descriptions

So:

• Fue un error terrible = it was a terrible mistake

Using estar here would sound wrong, because estar is normally used for:

• location
• temporary states
• resulting conditions

You can say:

• Estuvo terrible = it was terrible

But that is different. It means something like it was terrible, without classifying it specifically as a mistake.

Because error is a countable noun, and in Spanish singular countable nouns normally need an article or another determiner.

So:

• un error = a mistake/an error

You usually cannot say:

• Fue error terrible

That sounds incomplete or ungrammatical in normal Spanish.

The article un shows that it is one mistake, not a general abstract idea.

Because error is a masculine noun:

• el error
• un error

This is useful because English speakers sometimes try to guess gender from meaning, but in Spanish you must learn the noun’s gender with the word itself.

Also, nouns ending in -or are very often masculine:

• el error
• el color
• el doctor

Yes, you can say both:

• un error terrible
• un terrible error

But they are not exactly identical in feel.

The most neutral pattern in Spanish is often:

• noun + adjective

So un error terrible is very natural and straightforward.

If you put the adjective before the noun:

• un terrible error

it often sounds a bit more:

• emphatic
• expressive
• subjective
• literary or dramatic

So both are correct, but un error terrible is the more neutral wording.

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

English usually needs a subject:

• It was a terrible mistake
• That was a terrible mistake

Spanish often does not:

• Fue un error terrible

The subject is understood from the situation. Depending on context, it could mean:

• it was a terrible mistake
• that was a terrible mistake
• he/she/it was a terrible mistake in a very unusual context

In normal use, the listener understands what fue refers to from what was said before.

Yes, and in Spain this is a very useful contrast.

Both are possible, but they do not sound exactly the same:

• Fue un error terrible = preterite; the event is seen as a finished past event
• Ha sido un error terrible = present perfect; often used in Spain for something recent or still connected to the present

In Spain, you often hear the present perfect with time periods that are still considered part of the present:

• Este año ha sido un error terrible would be odd semantically, but grammatically shows the pattern
• Hoy ha sido un día largo
• Esta mañana ha sido un desastre

So if the speaker feels the mistake is recent or still relevant now, ha sido may sound more natural. If it is simply a completed past event, fue works very well.

Usually yes. In this sentence, terrible is clearly negative and means something like:

• very bad
• awful
• serious in a bad way

So un error terrible is a strong negative judgment.

Like in English, the exact force depends on context. But here it definitely does not mean anything positive.

A simple Spain Spanish approximation is:

• fue ≈ fweh or almost fway, but as one syllable
• un ≈ oon with a shorter vowel
• error ≈ eh-RROR with a rolled rr
• terrible ≈ te-RREE-bleh

A few useful points:

• fue is one syllable, not foo-ay
• the rr in error is a strong rolled sound
• r in terrible is also strong here because it starts a syllable after a consonant
• in Spain, e and o are pure vowels, not diphthongs like many English vowels

A rough full pronunciation guide:

• fweh oon eh-RROR teh-RREE-bleh

Not freely.

The normal order is:

• Fue un error terrible

This is the most natural way to say it.

Other orders are either:

• more marked
• more literary
• or simply unnatural in everyday speech

For example:

• Un error terrible fue sounds very unnatural by itself
• Terrible fue el error sounds highly literary or poetic

So for normal conversation, stick with:

• Fue un error terrible