Ella vertió la leche despacio.

Vertió is the third-person singular preterite of verter, which means to pour or to tip out a liquid.

In this sentence, it means she poured the milk.
It usually suggests that a liquid is being emptied or poured from one container into another, or out of something.

That’s true: Spanish often leaves out subject pronouns because the verb ending already tells you who the subject is.

So Vertió la leche despacio is also a perfectly natural sentence.

Including ella can:

• add emphasis
• create contrast with someone else
• make the subject extra clear in context

For example:

• Ella vertió la leche despacio, pero él la bebió rápido.

So in many situations, ella is optional.

In Spanish, it is very common to use the definite article with nouns where English might or might not use one.

Here, la leche means the milk. It refers to a specific milk involved in the situation.

If you just said leche, it would usually sound less complete here. With a direct object like this, Spanish normally uses the article:

• Compré la leche.
• Bebió el agua.
• Vertió la leche.

So la leche is the natural choice.

It is the preterite tense.

The preterite is used for a completed action in the past.
So virtió means that the action of pouring happened and was completed.

Compare:

• Ella vertió la leche = she poured the milk
• Ella vertía la leche = she was pouring the milk / she used to pour the milk

So in your sentence, the pouring is presented as a finished event.

The accent mark shows the correct stress.

Without the accent, vertio would not follow normal Spanish stress rules correctly.
With the accent, virtió is stressed on the final syllable: ver-TIÓ.

This is very common in the preterite:

• comió
• vivió
• salió
• virtió

So the accent is there because of the verb ending and pronunciation.

It is regular in the preterite, which is the tense used here:

• vertí
• vertiste
• virtió
• vertimos
• vertisteis
• vertieron

However, in some present-tense forms it has a stem change:

• vierto
• viertes
• vierte
• vertemos
• vertéis
• vierten

So a good way to think about it is:

• regular here
• not completely regular in all tenses

Despacio is an adverb. It means slowly.

It describes how she poured the milk.

Unlike adjectives, adverbs do not change for gender or number, so it stays despacio no matter who or what is involved:

• Ella habló despacio.
• Ellos caminaron despacio.
• Leí despacio.

Yes, it can go in different positions, but the version in your sentence is very natural:

• Ella vertió la leche despacio.

This is the most straightforward order:
subject + verb + object + adverb

Other positions are possible, depending on style or emphasis:

• Ella despacio vertió la leche. → possible, but less neutral
• Despacio, ella vertió la leche. → more literary or emphatic

For everyday Spanish, putting despacio at the end is very common and natural.

Yes. Both can mean slowly.

• despacio = very common, everyday, natural
• lentamente = also correct, but often sounds a bit more formal or deliberate

So:

• Ella vertió la leche despacio.
• Ella vertió la leche lentamente.

Both are correct, but despacio is usually the more conversational choice.

In most of Spain, ella is usually pronounced roughly like EH-ya.

A few notes:

• e = like e in met (but cleaner)
• ll is usually pronounced like a y sound in much of modern Spain
• the stress is on the first syllable: E-lla

So for many speakers, it sounds close to EH-ya.

In some regions, the ll sound may be pronounced differently, but EH-ya is a good practical guide for most learners.

Sometimes it can overlap with spilled, depending on context, but verter mainly means to pour.

If you specifically want to spill by accident, Spanish often uses:

• derramar = to spill
• se le cayó la leche = the milk fell / she dropped the milk

So:

• Vertió la leche = she poured the milk
• Derramó la leche = she spilled the milk

The exact meaning depends on context, but by itself virtió usually suggests pouring rather than accidentally spilling.

Sometimes yes, but they are not exactly the same.

• verter focuses on pouring out liquid
• echar is a much broader verb meaning things like to put, to throw, to add, or sometimes to pour

So:

• Vertió la leche = more specifically poured the milk
• Echó la leche = could mean poured/added the milk, depending on context

If you are talking specifically about the action of pouring liquid, verter is more precise.
If you are talking more generally about adding milk to something, echar is very common in everyday Spanish.