El viaje fue estupendo.

Because viaje is a masculine noun in Spanish, so it takes el in the singular.

• el viaje = the trip / the journey
• los viajes = the trips / the journeys

This is something you usually just have to learn with the noun. The ending -e does not reliably tell you whether a noun is masculine or feminine.

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

So here:

• El viaje = the trip
• fue = was
• estupendo = great / wonderful

Literally, fue means was here.

A useful thing to know: fue can also be a form of ir (to go), but in this sentence it is clearly from ser, because it is followed by an adjective (estupendo), not a destination or movement.

This is one of the most common questions English speakers have.

• fue = preterite
• era = imperfect

In El viaje fue estupendo, the speaker is treating the trip as a completed whole and giving an overall judgment about it: The trip was great.

Using era would sound less natural in most normal contexts, because era is more for:

• background description
• ongoing states in the past
• habitual past situations

So:

• El viaje fue estupendo = The trip was great.
  (a complete event, viewed as a whole)

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

Here, viaje is:

• masculine
• singular

So the adjective must also be:

• masculine
• singular

That gives:

• el viaje fue estupendo

Compare:

• la experiencia fue estupenda
• los viajes fueron estupendos
• las vacaciones fueron estupendas

Not usually in everyday translation.

Although estupendo and stupendous are related words, estupendo in Spanish often simply means:

• great
• wonderful
• excellent
• really good

So El viaje fue estupendo is usually best understood as:

• The trip was great
• The trip was wonderful

rather than the more dramatic English word stupendous.

In this sentence, estupendo is not directly placed after the noun as part of a noun phrase like un viaje estupendo.

Instead, it comes after the verb ser and works as a predicate adjective:

• El viaje = subject
• fue = linking verb
• estupendo = adjective describing the subject

This is the same basic pattern as:

• La casa es grande
• El libro fue interesante
• La comida estaba deliciosa

So the position is normal because the adjective is linked to the subject through the verb.

Yes, you may hear that, and it can sound natural. But there is a slight nuance.

• fue estupendo gives a general evaluation of the trip as a whole.
• estuvo estupendo can also mean it was great, but with a more event-like or experiential feel, similar to saying it went great or turned out great.

In many real conversations, both can work. But for a simple overall judgment about a completed trip, fue estupendo is very straightforward and standard.

No. In this sentence, fue must be understood as a form of ser, not ir.

Why?

Because estupendo is an adjective describing el viaje. The structure is:

• noun + ser
  • adjective

If fue were from ir, you would normally expect something involving movement or destination, for example:

• Fue a Madrid = He/She went to Madrid

So in El viaje fue estupendo, the meaning is clearly was, not went.

A simple pronunciation guide is:

el BYAH-heh fweh ehs-too-PEN-doh

A few notes:

• viaje is pronounced roughly BYAH-heh
• the j in viaje is the throaty Spanish sound, like the ch in Scottish loch
• fue is one syllable: fweh
• v and b are pronounced very similarly in Spanish, so viaje does not sound like English vy-

If you want a more detailed approximation for Spain:

• El viaje fue estupendo ≈ el BYAH-heh fweh es-too-PEN-doh

No, not in normal Spanish.

You need the definite article el here because viaje is a specific noun phrase: the trip.

So:

• El viaje fue estupendo = correct

Without the article, Viaje fue estupendo sounds ungrammatical in standard Spanish.

Grammatically, yes, but it would usually need a special context.

For example, era estupendo could make more sense if you were describing the trip in progress or giving background in a longer story, such as:

• Al principio, el viaje era estupendo, pero luego empezó a llover.
  At first, the trip was great, but then it started to rain.

So era is possible when you want an ongoing past description, but as a simple finished summary, fue estupendo is the more natural choice.