El pasado fue diferente.

Because pasado is acting as a noun here: el pasado = the past.

In Spanish, abstract ideas often use the definite article where English may or may not. So:

• el pasado = the past
• el presente = the present
• el futuro = the future

Without el, the sentence would sound incomplete in normal Spanish.

Here, pasado is a noun.

Even though pasado can also be:

• a past participle of pasar
• an adjective meaning past or last in some contexts

in this sentence, el pasado is a fixed expression meaning the past.

This is a very common question.

Both fue and era can translate as was, but they do not feel the same in Spanish.

• fue = preterite; presents it as a complete fact, often seen as a whole
• era = imperfect; gives background, description, or an ongoing past state

So:

• El pasado fue diferente.
  = The past was different.
  This sounds like a completed statement or conclusion.

• El pasado era diferente.
  = The past was different.
  This sounds more descriptive, reflective, or like setting the scene.

Both are possible, but fue gives a more definite, self-contained statement.

Because Spanish usually uses ser to describe what something was like by nature or character, and estar for states or conditions.

Here, different is describing the past in a broad, essential way, so ser is the natural choice:

• El pasado fue diferente.

Using estuvo diferente would sound unusual in this context, because it suggests a temporary condition, and that does not fit well with the past as a concept.

Yes. Fue is an irregular preterite form.

It is the third-person singular form of ser in the preterite:

• yo fui
• tú fuiste
• él/ella/usted fue

A tricky point: these same preterite forms are also used for ir:

• fui, fuiste, fue...

So fue can mean either was or went, depending on context. In this sentence, it clearly means was.

Because in Spanish, adjectives usually come after the noun.

So:

• el pasado diferente would mean the different past and would need a bigger phrase to sound natural
• El pasado fue diferente means The past was different

Here, diferente is not directly attached to the noun inside the noun phrase; it comes after fue as part of the predicate, which is completely normal.

It changes for number, but not for gender in the singular.

So you get:

• diferente = singular
• diferentes = plural

Examples:

• El pasado fue diferente.
• Las épocas pasadas fueron diferentes.

You do not say diferenta. The form stays diferente for both masculine and feminine singular.

Fue is pronounced as one syllable.

A rough guide for an English speaker is:

• fweh

Important points:

• The u is pronounced because it is part of the diphthong ue
• It is not two syllables like fu-e
• There is no written accent because it is monosyllabic

So the full sentence is roughly:

• el pa-SA-do fwe di-fe-REN-te

Not naturally in this meaning.

Estaba diferente would usually suggest a temporary or situational change in how something seemed, and with el pasado, that sounds odd.

For a broad statement like this, Spanish prefers:

• El pasado fue diferente
• or sometimes El pasado era diferente, depending on nuance

So for learners, ser is the safe and natural choice here.

Normally, no.

Spanish needs the article here because pasado is being used as a noun meaning the past. The standard expression is:

• el pasado

So the natural sentence is:

• El pasado fue diferente.

Leaving out el would sound incomplete or ungrammatical in normal usage.

Yes. The noun phrase is el pasado, and it is conventionally masculine in Spanish.

That is why it takes:

• el, not la
• fue, which does not show gender
• and an adjective like diferente, which in the singular does not visibly change for gender anyway

This is something you mostly just learn as part of the fixed expression el pasado.

Yes, definitely.

That version means something like:

• In the past, everything was different

It is a longer sentence with a slightly different structure, but it is very natural.

Compare:

• El pasado fue diferente. = a statement about the past itself
• En el pasado, todo fue diferente. = a statement about what things were like in past times

So the original sentence is shorter and more abstract.