La investigación empezó ayer.

Because Spanish normally uses an article with a noun in sentences like this. La investigación means the investigation or the research.

In this sentence, it sounds like a specific investigation/research that the speaker has in mind. Without la, the sentence would not sound natural in normal Spanish.

Investigación is feminine, so it takes la.

A very useful pattern is that nouns ending in -ción are almost always feminine:

• la información
• la situación
• la investigación

So if you see -ción, feminine is usually a safe guess.

Empezó is the preterite tense, third person singular, from empezar.

So it means he/she/it started or began.
Here, the subject is la investigación, so empezó means the investigation started.

The preterite is used because the sentence talks about a completed event at a specific time in the past: it started yesterday.

That is exactly the kind of situation where Spanish often uses the preterite:

• empezó ayer = it started yesterday
• terminó anoche = it finished last night

If you used the imperfect, empezaba, it would usually suggest background, repetition, or something in progress, which does not fit as well here.

In Spain, the present perfect (ha empezado) is often used for recent past events connected to the present, especially with time expressions like hoy.

But with ayer, Spanish normally uses the preterite:

• Hoy ha empezado
• Ayer empezó

So La investigación empezó ayer is the standard choice.

Yes, but only in certain tenses and forms.

In the present tense, it has a stem change:

• empiezo
• empiezas
• empieza

But in the preterite, that stem change does not happen for this -ar verb:

• empecé
• empezaste
• empezó

Notice one spelling change in empecé: the z changes to c before e. That is just to preserve the sound. In empezó, the z stays because it comes before ó.

The accent marks show where the stress goes.

• investigación is stressed on the last syllable: in-ves-ti-ga-CIÓN
• empezó is stressed on the last syllable: em-pe-ZÓ

Without the accent marks, Spanish spelling rules would make you stress them differently.

So the accents are not optional; they are part of the correct spelling.

No. Ayer is used directly, without a preposition.

So you say:

• empezó ayer
• llegó ayer
• salimos ayer

Not:

• en ayer
• a ayer

You only add something if the meaning changes:

• desde ayer = since yesterday
• hasta ayer = until yesterday

Yes. Spanish word order is flexible.

You can say:

• La investigación empezó ayer
• Ayer empezó la investigación

Both are correct. The difference is mainly emphasis:

• La investigación empezó ayer is neutral.
• Ayer empezó la investigación puts more focus on yesterday.

Yes. Investigación can mean either one, depending on context.

For example:

• police/legal context: investigation
• academic/scientific context: research

So this sentence could mean:

• The investigation started yesterday
• The research started yesterday

The surrounding context tells you which meaning is intended.

Yes. Comenzó and empezó both mean started/began here.

• La investigación empezó ayer
• La investigación comenzó ayer

Both are natural. Empezar is often a little more everyday and common in speech, while comenzar can sound slightly more formal in some contexts, but the difference is small.

A simple approximation is:

lah im-bes-tee-gah-THYON em-peh-THOH ah-YEHR

A few notes for Spain Spanish:

• the z in empezó sounds like th in thin
• the ci in investigación also sounds like th
• the stress is on -ción and -zó

So the strongest syllables are:

• inves-tiga-CIÓN
• empe-ZÓ