Por la noche, pongo un vaso de agua sobre la mesilla.

In Spanish, por la noche is the normal way to say at night / in the evening / at nighttime in a general time-expression sense.

• por la mañana = in the morning
• por la tarde = in the afternoon / evening
• por la noche = at night / in the evening

En la noche does exist, but it is less common in Spain for this everyday meaning. It can sound more literary, regional, or more tied to a specific night/context. For standard Spain Spanish, por la noche is the safest choice here.

Because Por la noche has been moved to the front of the sentence as a time expression.

The comma is not absolutely required in all cases, but it is very common and natural when a short introductory phrase comes first:

• Por la noche, pongo...

It helps separate the time phrase from the main statement. Without the comma, the sentence would still usually be understood, but the comma makes it cleaner and more readable.

Spanish often leaves out subject pronouns because the verb ending already shows who the subject is.

• pongo = I put / I place
• pones = you put
• pone = he/she/it puts

So pongo already tells you the subject is I, and yo is unnecessary unless you want emphasis or contrast.

For example:

• Yo pongo un vaso de agua, pero él no.
  = I put a glass of water, but he doesn’t.

Pongo is the first-person singular present tense form of poner.

• infinitive: poner = to put / to place
• yo pongo = I put / I place
• tú pones = you put
• él/ella pone = he/she puts

So in this sentence, pongo means I put or I place.

Because poner is an irregular verb.

Many very common Spanish verbs are irregular, especially in the yo form. Poner follows a pattern like:

• tener → tengo
• venir → vengo
• poner → pongo

So pongo is just the correct irregular yo form and needs to be learned as part of the verb’s conjugation.

Yes. The Spanish present tense can describe:

• what you do now
• what you do generally
• habits or routines

So pongo here can naturally mean something like I put as part of a regular nightly habit.

This is very similar to English simple present:

• At night, I put a glass of water on the bedside table.

This is a very common Spanish structure:

container + de + contents

So:

• un vaso de agua = a glass of water
• una taza de café = a cup of coffee
• una botella de vino = a bottle of wine

Literally, it is a glass of water, not a water glass in the English sense of a type of glass.

Because after a container or quantity expression, Spanish normally uses de + noun without an article when speaking generally.

So:

• un vaso de agua
• una taza de té
• un kilo de arroz

You would add an article only if you were referring to something specific:

• un vaso del agua fría que preparaste
• a glass of the cold water you prepared

But in the basic sentence, de agua is the normal form.

Because sobre means on or on top of, which fits the idea of placing the glass on the surface of the bedside table.

• sobre la mesilla = on the bedside table
• en la mesilla would suggest in the bedside table, such as inside a drawer or compartment

You could also hear:

• encima de la mesilla

That also means on top of the bedside table.
In this sentence, sobre la mesilla is perfectly natural and a bit more compact.

In Spain, mesilla usually means a small table, and very often it refers to a bedside table / nightstand.

So la mesilla here will normally be understood as the little table next to the bed.

This is a good example of regional vocabulary. In other Spanish-speaking places, people might prefer expressions like:

• mesita de noche
• mesa de noche

But mesilla is very typical in Spain.

Yes, but the nuance changes slightly.

• pongo focuses on the action of putting/placing
• dejo focuses more on leaving something somewhere

So:

• Pongo un vaso de agua sobre la mesilla
  = I put/place a glass of water on the bedside table.

• Dejo un vaso de agua sobre la mesilla
  = I leave a glass of water on the bedside table.

Both can be natural, but pongo is more about placement, while dejo is more about the result of leaving it there.

Yes. Spanish word order is flexible.

You could also say:

• Pongo un vaso de agua sobre la mesilla por la noche.

That is still correct. The difference is mainly one of emphasis:

• Por la noche, pongo... emphasizes when
• Pongo... por la noche sounds a bit more neutral and leaves the time information until the end

Putting the time expression first is very common when setting the scene.