Mi verŝas akvon en la glason.

Because akvon is the direct object of the verb verŝas.

In Esperanto, the direct object usually takes the ending -n. Here:

• mi = the subject, the person doing the action
• verŝas = pours / am pouring
• akvon = water, the thing being poured

So akvo becomes akvon because it is what the action is being done to.

Good question. Here the -n is not marking a direct object; it is showing direction or movement toward a place.

With en:

• en la glaso = in the glass, inside the glass
• en la glason = into the glass, with movement into it

So in Mi verŝas akvon en la glason, the water is moving into the glass. That is why glaso becomes glason.

The difference is:

• en la glaso = in the glass (location, no movement)
• en la glason = into the glass (movement toward the inside)

Examples:

• La akvo estas en la glaso. = The water is in the glass.
• Mi verŝas akvon en la glason. = I pour water into the glass.

This is a very important Esperanto pattern: -n can show motion toward a place.

Verŝi means to pour.

In many contexts, it can also suggest to spill or to cause liquid to flow out, depending on the situation. But in this sentence, it clearly means to pour.

Related forms:

• verŝi = to pour
• verŝas = pours / am pouring / do pour
• verŝis = poured
• verŝos = will pour

Because -as is the Esperanto ending for the present tense.

So:

• verŝi = infinitive, to pour
• verŝas = present, pour / am pouring
• verŝis = past, poured
• verŝos = future, will pour

Esperanto verbs are very regular, so once you learn the endings, they are easy to recognize.

Because Esperanto usually expresses tense directly in the verb ending itself.

So verŝas already means something like:

• pour
• am pouring
• do pour

The exact English translation depends on context. Esperanto does not need a separate helping verb like am here.

Mi is the Esperanto word for I.

Esperanto personal pronouns are fixed forms:

• mi = I
• vi = you
• li = he
• ŝi = she
• ĝi = it
• ni = we
• ili = they

Since mi is the subject of the sentence, it does not take -n.

La is the definite article, meaning the.

So:

• glason = a glass / glass as an object, depending on context
• la glason = the glass, a specific glass

Esperanto has only one definite article: la. It does not change for gender, number, or case. Even when the noun gets -n, the article stays la:

• la glaso
• la glason

No. Esperanto has no indefinite article.

So:

• glaso can mean a glass or just glass, depending on context
• la glaso means the glass

That means Esperanto often leaves English a/an untranslated.

Yes. That would mean I am pouring the water into the glass, with emphasis on a specific water.

Compare:

• Mi verŝas akvon en la glason. = I am pouring water into the glass.
  (water in a general sense, or some water)
• Mi verŝas la akvon en la glason. = I am pouring the water into the glass.
  (a specific water already known from context)

Both are grammatical; they just differ in specificity.

Because en means in / into, while al means to / toward.

Here the important idea is that the water goes inside the glass, so en is the natural choice:

• en la glason = into the glass

If you used al, it would mean only movement toward the glass, not necessarily inside it.

Yes, Esperanto word order is fairly flexible, because endings show grammatical roles.

The most neutral order is:

• Mi verŝas akvon en la glason.

But other orders can be possible for emphasis, such as:

• Akvon mi verŝas en la glason.
• En la glason mi verŝas akvon.

Even so, learners should usually start with the normal subject-verb-object order, because it is the clearest and most natural in everyday use.

Yes, akvo is singular, and here it refers to water as a substance, not to separate countable items.

In Esperanto, just like in English, some nouns are often used as mass nouns:

• akvo = water
• lakto = milk
• sablo = sand

You would not normally say akvoj unless you meant different kinds of waters, such as mineral waters or waters from different places.