El médico explicó la operación.

Because médico is a singular masculine noun, so it takes the masculine singular definite article el.

In this sentence, el médico means the doctor. Spanish usually uses articles very regularly, often more than English does.

Because operación is being used as a specific noun: the operation. In Spanish, it is very normal to include the definite article with a specific thing.

Also, operación is a feminine singular noun, so it takes la.

A very common pattern in Spanish is that nouns ending in -ción are feminine.

So:

• la operación
• la explicación
• la información

That pattern is very reliable and useful to remember.

Explicó is the preterite form of explicar. It means that the action was completed in the past.

More specifically, it is:

• third person singular
• preterite
• from explicar

So explicó means he explained or she explained, depending on context.

Because Spanish often leaves out subject pronouns when they are not needed. The verb ending already tells you the person and number.

Here, explicó already shows a third person singular subject, so él is unnecessary unless you want extra emphasis or contrast.

Spanish does this much more often than English.

Because la operación is the direct object, and it is a thing, not a person.

The so-called personal a is generally used before specific human direct objects, not before things.

So:

• El médico explicó la operación. = no a
• El médico vio al paciente. = a is used because paciente is a person

Also, with explicar, if you say explain something to someone, the a would introduce the person receiving the explanation:

• El médico le explicó la operación al paciente.

They show where the stress falls.

• médico → stress on mé
• explicó → stress on the final ó
• operación → stress on the final ó

This is especially important because Spanish stress is very regular, and accent marks tell you when a word does not follow the default pattern.

They are also part of the correct spelling, so you should write them.

A rough pronunciation is:

el MEH-dee-koh eks-plee-KOH lah oh-peh-rah-THYON

In much of Spain:

• c before i or e sounds like th in think
• so operación sounds like oh-peh-rah-THYON

A more careful breakdown:

• El médico → el MEH-dee-koh
• explicó → eks-plee-KOH
• la operación → lah oh-peh-rah-THYON

Yes, but not in the same grammatical role.

Here, el médico is a noun, meaning the doctor.

But médico can also be an adjective:

• el personal médico = the medical staff
• un problema médico = a medical problem

So the word can function as either a noun or an adjective, depending on the sentence.

Yes. El doctor explicó la operación is also possible.

In many contexts, doctor and médico can both mean doctor, but médico is often a little more neutral and directly tied to the profession, while doctor can sometimes sound more like a title.

Both are very common.

Yes, although El médico explicó la operación is the most neutral and straightforward order: subject + verb + object.

Spanish allows more flexibility than English. For example:

• La operación la explicó el médico.

This changes the emphasis, making la operación more prominent.

But for a learner, the original order is the safest and most natural one to use first.

In standard modern Spanish, especially in Spain, you would usually say:

La médica explicó la operación.

Here:

• la = feminine article
• médica = feminine form of médico

So the noun changes to match the gender of the person being described.