El sargento llegó ayer.

Because sargento here is an ordinary noun meaning a specific person, so Spanish normally uses the definite article el for the sergeant.

• el sargento = the sergeant
• un sargento = a sergeant

You can leave the article out in some special cases, especially when directly addressing someone:

• Sargento, venga aquí. = Sergeant, come here.

But in a normal statement like this one, el is the natural choice.

Spanish often drops subject pronouns when they are not needed. The verb ending already tells you the subject is third person singular.

• llegó = he arrived / she arrived / it arrived / you arrived (formal singular, depending on context)

Since el sargento is already there, Spanish does not need él.

A fuller version would be possible, but it usually sounds unnecessary:

• El sargento llegó ayer. = normal
• Él llegó ayer. = possible, but now he is being emphasized

Llegó is the preterite of llegar.

More exactly, it is:

• third person singular
• preterite
• from the verb llegar = to arrive

The preterite is used for a completed action. Since ayer means yesterday, the arrival is seen as a finished event, so the preterite is exactly what you would expect.

Llegar is a regular -ar verb in the preterite.

The relevant endings are:

• llegué = I arrived
• llegaste = you arrived
• llegó = he/she arrived
• llegamos = we arrived
• llegasteis = you all arrived
• llegaron = they arrived

So:

• stem: lleg-
• ending for él/ella/usted in the preterite: -ó
• result: llegó

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

• lle-GÓ

It also helps distinguish it from llego, which would mean I arrive in the present tense:

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

So the accent is not optional here; it changes both pronunciation and meaning.

A rough Spain-Spanish pronunciation is:

el sar-HEN-to ye-GO a-YER

A few helpful details:

• sargento: the g before e sounds like the strong Spanish j, like the ch in Scottish loch
• llegó: in most of Spain, ll sounds like y in yes
• ayer: the y also sounds like y in yes
• the final r in ayer is a light tap

A more technical pronunciation would be roughly:

[el saɾˈxento ʝeˈɣo aˈʝeɾ]

Yes, it could go somewhere else. Spanish word order is flexible, especially with time expressions like ayer.

All of these are possible:

• El sargento llegó ayer.
• Ayer llegó el sargento.
• El sargento ayer llegó. (less neutral, more marked)

The version with ayer at the end is very natural and neutral. Putting ayer first often gives it a little more emphasis, as in Yesterday, the sergeant arrived.

With ayer, standard Spanish normally uses the preterite:

• llegó ayer

In Spain, ha llegado is very common for recent past connected to the present, especially with time expressions like:

• hoy
• esta mañana
• esta semana

But ayer refers to a finished time period, so llegó is the expected form.

So in standard usage:

• Hoy ha llegado. = natural
• Ayer llegó. = natural

Because llegó presents the arrival as a single completed event.

• llegó = arrived
• llegaba = was arriving / used to arrive

With ayer, if you simply want to say that the sergeant arrived yesterday, llegó is the normal choice.

Llegaba would need a different context, for example as background in a story:

• Ayer, cuando sonó el teléfono, el sargento llegaba al cuartel.

That is much less neutral and more narrative.

In this sentence, el shows that the person is being treated grammatically as masculine.

If the sergeant were female, you would normally change the article:

• La sargento llegó ayer.

You may also hear la sargenta in some usage, but la sargento is very common, especially in military contexts.

So the key change is usually the article:

• el sargento = the male sergeant
• la sargento = the female sergeant

Not really in normal context. El sargento llegó ayer is a straightforward past statement: a specific sergeant arrived yesterday.

The only small ambiguity is in who the sergeant is, not in the grammar. If the context has already identified a particular sergeant, then el sargento means the sergeant. The verb form itself clearly marks a completed past action.