Mi metas akvon en glason antaŭ la tagmanĝo.

Because akvon is the direct object of metas.

In Esperanto, the thing directly affected by the verb usually gets -n. Here:

• Mi = I
• metas = put / place
• akvon = water

So akvon is the thing being put.

Compare:

• Mi trinkas akvon. = I drink water.
• Mi metas akvon en glason. = I put water into a glass.

In both sentences, akvon is what the action is acting on, so it takes -n.

Here -n shows movement into something.

Normally, en means in. But when there is movement to the inside of a place/container, Esperanto often uses en + -n to show into.

So:

• en glaso = in a glass
• en glason = into a glass

That is why:

• Mi metas akvon en glason. = I put water into a glass.

A useful contrast:

• La akvo estas en glaso. = The water is in a glass.
  No movement, so no -n.
• Mi verŝas la akvon en glason. = I pour the water into a glass.
  Movement into it, so -n.

Because here antaŭ is just a preposition meaning before, and its noun stays in the normal form.

So:

• antaŭ la tagmanĝo = before the lunch / before lunch

There is no direct object here, and no accusative ending is needed.

In Esperanto, prepositions normally take a noun without -n, unless there is some special reason to use the accusative, such as direction in certain expressions. That is not happening here.

So:

• Mi metas akvon en glason antaŭ la tagmanĝo.
  = I put water into a glass before lunch.

La is the Esperanto definite article, meaning the.

So la tagmanĝo means:

• the lunch
• or the specific lunchtime being talked about

Esperanto has la, but it does not have an indefinite article like a or an.

That means:

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

In this sentence, la tagmanĝo makes the meal sound more specific. Depending on context, you might also hear antaŭ tagmanĝo, which can sound more general, like before lunch in general.

Because Esperanto has no indefinite article.

English distinguishes:

• a glass
• the glass

Esperanto only marks the definite one:

• glaso = a glass / glass
• la glaso = the glass

So en glason naturally means into a glass if no specific glass has already been identified.

This is very normal in Esperanto.

Metas comes from the verb meti, which means to put, to place, or to set.

The ending -as marks the present tense:

• meti = to put
• metas = put / am putting / puts
• metis = put / was putting
• metos = will put

So:

• Mi metas = I put / I am putting

One extra note: because the object is water, an English speaker might expect pour rather than put. In Esperanto, meti is still understandable, but if you want to emphasize pouring liquid, verŝi may sound more natural:

• Mi verŝas akvon en glason. = I pour water into a glass.

Yes, it is one word, and it is a very typical Esperanto compound.

• tago = day
• manĝo = meal / eating
• tagmanĝo = lunch

Literally, it is something like day-meal.

Esperanto often builds vocabulary this way. Some related examples:

• matenmanĝo = breakfast
• vespermanĝo = dinner / supper
• manĝi = to eat

This compound style is very common and useful in Esperanto.

Yes, to some extent.

Esperanto word order is more flexible than English because endings like -n help show what each word is doing.

The most neutral order here is:

• Mi metas akvon en glason antaŭ la tagmanĝo.

But you could move parts around for emphasis, for example:

• Antaŭ la tagmanĝo mi metas akvon en glason.
  This emphasizes the time.
• Akvon mi metas en glason antaŭ la tagmanĝo.
  This emphasizes water.

Even with different word order, akvon is still the direct object because of -n.

That said, the original version is the most straightforward and natural for a learner.

A few pronunciation points in this sentence are especially useful:

• aŭ in antaŭ is a diphthong, roughly like ow in now
• ĝ in manĝo sounds like the j in judge
• ĉ is not in this sentence, but many learners confuse it with ĝ; ĉ sounds like ch in church

So roughly:

• antaŭ ≈ ahn-TOW
• manĝo ≈ MAHN-jo

Also remember that Esperanto stress normally falls on the second-to-last syllable:

• an-TAŬ
• MAN-ĝo
• gla-SON
• tag-MAN-ĝo

In Esperanto, mass nouns like akvo still behave like ordinary singular nouns grammatically.

So you say:

• akvo = water
• akvon = water as a direct object

You do not need any special grammar for uncountable nouns.

If you make it plural, akvoj, that usually means different kinds of water or waters, not just more water.

So in this sentence, akvon is exactly what you would expect: singular noun + accusative ending.