El siniestro fue ayer.

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

Here it is used because the sentence refers to an event at a finished point in the past: ayer. With a clearly completed time like yesterday, Spanish normally uses the preterite.

So the structure is:

• El siniestro = subject
• fue = was
• ayer = yesterday

Because fue presents the event as a completed fact.

Spanish often contrasts:

• fue = completed event, seen as a whole
• era = background description, ongoing situation, or habitual past

Since ayer points to a specific finished time, fue is the natural choice.

A learner shortcut:

• What happened? → often preterite: fue
• What was it like / what was going on? → often imperfect: era

It can be either, depending on context.

• fue = was from ser
• fue = went from ir

In this sentence, it must be from ser, because el siniestro is not something that went somewhere. The sentence is identifying when the incident happened, so fue means was.

In this sentence, siniestro is a noun, not an adjective.

In Spain, it is often used in formal, official, journalistic, or insurance language to refer to an accident, damaging incident, or loss event. It can sound more formal than everyday words like accidente.

So a native speaker might hear siniestro especially in:

• news reports
• police language
• insurance documents

El is the definite article, so it suggests a specific incident already known from context.

Spanish uses definite articles very naturally when both speaker and listener know what is being talked about.

Compare:

• El siniestro fue ayer. = a specific incident already identified
• Hubo un siniestro ayer. = there was an incident yesterday, introducing it for the first time

Yes. Spanish word order is flexible.

Both are grammatical, but the emphasis changes a little:

• El siniestro fue ayer. = neutral, starting with the event
• Ayer fue el siniestro. = puts more focus on when

Spanish often moves time expressions like ayer for emphasis or style.

Because ayer is an adverb of time, so it usually appears by itself.

You say:

• hoy
• ayer
• mañana

without a preposition.

So:

• fue ayer is correct
• not fue en ayer

Yes.

The sentence breaks down like this:

• El siniestro = subject
• fue = verb
• ayer = time expression

So it is a very straightforward sentence pattern: subject + verb + time expression

No. It can also be an adjective.

As an adjective, siniestro can mean something like sinister, ominous, or darkly threatening. But in this sentence it is clearly a noun meaning an incident or accident.

So here:

• el siniestro = noun phrase

not an adjective describing something else.

A simple pronunciation guide is:

el si-NIES-tro fue a-YER

A few useful points:

• siniestro is stressed on -nies-
• fue is usually pronounced as one syllable
• ayer is stressed on the second syllable: a-YER

In connected speech, fue ayer may sound quite smooth and fast together.