El coronel llegó ayer.

Because coronel is a noun, and here it means the colonel as a specific person.

So:

• el = the
• coronel = colonel

Spanish usually uses the definite article with specific singular nouns in this kind of sentence, just as English does here.

Llegó is the third-person singular preterite of llegar.

That means it can mean:

• he arrived
• she arrived
• it arrived
• you arrived (formal singular, usted)

In this sentence, the subject is El coronel, so llegó means the colonel arrived.

The preterite is used for a completed action in the past.

Here, ayer means yesterday, which is a finished time period, so the preterite is the natural choice:

• llegó ayer = arrived yesterday

This is different from forms like:

• llegaba = was arriving / used to arrive
• ha llegado = has arrived

With ayer, Spanish normally uses the preterite: llegó.

The accent mark shows that the stress falls on the last syllable:

• lle-GÓ

It also helps distinguish it from llego (without the accent), which is a different form:

• llego = I arrive / I am arriving
• llegó = he/she arrived

So the accent is both a pronunciation guide and a grammatical clue.

A good approximation is ye-GO.

A few useful points:

• ll in most of Spain is pronounced like the y in yes
• g before o is a hard g, like in go
• the stress is on the last syllable because of the accent: lle-GÓ

So: ye-GO

In careful phonetic terms, many speakers pronounce it roughly as /ʝeˈɣo/.

It is pronounced roughly ko-ro-NEL.

Helpful details:

• c before o sounds like k
• the r here is the single r sound, not the strong rolled rr
• the stress is on the last syllable: co-ro-NEL

So the whole sentence sounds approximately like:

el ko-ro-NEL ye-GO a-YER

Because ayer is an adverb, and it does not need a preposition.

So Spanish says:

• ayer = yesterday

not:

• en ayer
• a ayer

Just use ayer by itself.

Yes. Spanish word order is flexible.

You can say:

• El coronel llegó ayer.
• Ayer llegó el coronel.

Both are correct. The difference is mainly one of emphasis:

• El coronel llegó ayer focuses first on the colonel
• Ayer llegó el coronel highlights yesterday

In standard Spanish, ayer normally goes with the preterite, not the present perfect.

So the natural form is:

• llegó ayer

not:

• ha llegado ayer

This is especially important in Spain, where the present perfect is common with time expressions connected to the present, such as hoy, but ayer is still a finished time period, so llegó is the expected choice.

Yes, if the context already makes it clear who arrived.

Spanish often drops the subject when it is understood:

• Llegó ayer. = He/She arrived yesterday.

But in your sentence, El coronel is included to make the subject explicit.

In this sentence, coronel is masculine, so it takes el:

• El coronel

If the colonel is a woman, in Spain you will often hear:

• La coronel llegó ayer.

So the article changes to match the person’s gender, while the noun coronel often stays the same.