El monstruo desapareció.

It is the preterite (also called the simple past) of desaparecer.

• desaparecer = to disappear
• desapareció = he disappeared / she disappeared / it disappeared

In El monstruo desapareció, the preterite shows a completed action in the past: the monster disappeared.

Because it is the 3rd person singular preterite form of an -er verb.

The verb is desaparecer, and the preterite endings for regular -er verbs include:

• yo desaparecí
• tú desapareciste
• él/ella/usted desapareció
• nosotros desaparecimos
• vosotros desaparecisteis
• ellos/ellas/ustedes desaparecieron

So desapareció matches el monstruo, which is 3rd person singular.

The accent mark is part of the correct written form of the preterite ending -ió.

It shows that the stress falls on the last syllable:

• de-sa-pa-re-CIÓ

Without the accent, it would not be the correct spelling of this verb form.

El means the.

So:

• el monstruo = the monster

It is the masculine singular definite article, used with a masculine singular noun like monstruo.

El monstruo means the monster, while un monstruo means a monster.

The choice depends on whether the monster is seen as:

• specific/known → el monstruo
• non-specific/unknown → un monstruo

So this sentence is talking about a particular monster: the monster disappeared.

In Spanish, nouns have grammatical gender. Monstruo is a masculine noun, so it takes el.

• el monstruo

This does not necessarily mean the monster is biologically male. Grammatical gender and real-world sex are not always the same thing.

In most of Spain, the c before i is pronounced like the th in thin.

So desapareció sounds roughly like:

• de-sa-pa-re-thyó

A rough full pronunciation of the sentence in Spain would be:

• el monstruo desapareció
• roughly: el mons-TROO-o de-sa-pa-re-THYOH

In many Latin American accents, that same c would sound like an s, so -ció would sound more like syó.

Because Spanish often doesn’t need subject pronouns when the subject is already clear.

Here, the subject is explicitly stated:

• El monstruo desapareció

Adding él would usually be unnecessary. Spanish commonly leaves out subject pronouns unless there is a reason to emphasize or contrast them.

Yes. Spanish word order is more flexible than English.

Both are possible:

• El monstruo desapareció
• Desapareció el monstruo

The first is the more neutral subject + verb order.
The second can sound more like you are presenting the event first or shifting the focus slightly to the action.

Both mean The monster disappeared.

The infinitive is desaparecer.

That is the dictionary form of the verb:

• desaparecer = to disappear

So if you want to look it up, you would look for desaparecer, not desapareció.

It is mostly regular, but it has a spelling change in some forms.

For example:

• yo desaparezco = I disappear

That zc appears in the first person singular present because the verb ends in -ecer / -ocer / -ucir-type patterns that often do this.

But in the preterite form here, it behaves regularly:

• desapareció

So this sentence does not show the spelling change.

Because desapareció by itself only tells you that the subject is 3rd person singular. Spanish often does not specify gender in the verb form.

Here, the noun el monstruo tells you the subject is the monster, so in English the most natural translation is:

• The monster disappeared

If the subject were just omitted, desapareció could mean:

• he disappeared
• she disappeared
• it disappeared
• you disappeared (formal usted, depending on context)

Context tells you which one is meant.

Not exactly.

• desapareció = disappeared / a completed event
• desaparecía = was disappearing / used to disappear / disappeared in an ongoing or habitual sense, depending on context

So:

• El monstruo desapareció = the monster disappeared (one completed event)
• El monstruo desaparecía = the monster was disappearing / used to disappear

For this sentence, desapareció is the natural choice if you mean one finished event in the past.