El general llegó ayer.

El is the masculine singular definite article, meaning the.

So:

• el general = the general

Here, general is a noun meaning a military rank, not an adjective.

In this sentence, general refers to a male person, so el is used.

• el general = the general (male)
• la general could be used for a female general, though context matters and usage can vary

Also, even apart from real-life gender, Spanish nouns are assigned grammatical gender, and the article has to match.

Llegó is the preterite of llegar in the third person singular.

That means it translates as:

• he arrived
• she arrived
• it arrived
• or, as here, the general arrived

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

The accent mark shows that the stress falls on the last syllable: lle-GÓ.

It also helps distinguish this form from other forms:

• llegó = arrived
• llego = I arrive / I am arriving would actually be llego in the present tense

So the accent is important both for pronunciation and meaning.

Spanish often omits subject pronouns because the verb ending already tells you who the subject is.

Here, llegó already shows third person singular, so Spanish does not need él.

• El general llegó ayer. = normal
• Él llegó ayer. = He arrived yesterday
• Él, el general, llegó ayer. = possible, but more emphatic or contrastive

In this sentence, el general itself is the subject, so adding él would usually be unnecessary.

Because ayer refers to a finished time period: yesterday.

In Spanish, especially in standard usage, a finished time expression like ayer normally goes with the preterite:

• El general llegó ayer. = correct and standard

Using ha llegado with ayer is generally not the usual choice in standard Spanish. In Spain, the present perfect is common for recent past events connected to the present, but ayer still strongly favors the preterite.

Yes. Spanish word order is fairly flexible.

These are all possible:

• El general llegó ayer.
• Ayer llegó el general.

Both are natural. The difference is mainly focus:

• El general llegó ayer: neutral statement
• Ayer llegó el general: puts more emphasis on yesterday

Because general is a common noun, not a proper name.

In Spanish, job titles and ranks are usually written in lowercase unless they are part of a formal name or appear at the beginning of a sentence.

So:

• el general llegó ayer = lowercase is normal
• General López llegó ayer = General may be capitalized depending on style if used as part of the person’s title with a name

A simple approximation is:

• el → like el
• general → the g before e is a strong Spanish sound, not an English g
• llegó → roughly ye-GO
• ayer → roughly a-YER

A few useful pronunciation notes:

• The g in general is pronounced like the Spanish sound heard in words such as gente, not like the g in go
• The stress in llegó is on the final syllable because of the accent mark
• In most of Spain, ll and y are pronounced the same or very similarly

No. In natural English, has arrived yesterday is not correct.

The Spanish sentence El general llegó ayer means simply:

• The general arrived yesterday

The verb form llegó is a completed past action, so the best English match is the simple past.