Pongo el vaso sobre el posavasos y la botella al lado.

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

• poner = to put
• pongo = I put / I am putting

So the sentence starts with Pongo... because the subject is I.

This verb is irregular in the yo form:

• yo pongo
• tú pones
• él/ella pone

So you just have to learn pongo as part of the verb pattern.

Because Spanish often omits a repeated verb when it is easy to understand.

So:

• Pongo el vaso sobre el posavasos y la botella al lado

really means:

• Pongo el vaso sobre el posavasos y pongo la botella al lado

In English, we do something similar:

• I put the glass on the coaster and the bottle next to it

We understand that put applies to both objects.

Here, sobre means on / on top of.

So:

• sobre el posavasos = on the coaster

It suggests physical contact: the glass is resting on the coaster.

A learner might compare it with en:

• en = in / on / at, depending on context
• sobre is more specifically on top of / over

In this sentence, sobre is a natural choice because the glass is placed on top of the coaster.

Al lado means to the side or, in many contexts, next to it.

In this sentence:

• la botella al lado = the bottle next to it / at the side

It tells you where the bottle is being placed.

Very often, you will also hear the fuller structure:

• al lado de... = next to...

For example:

• La botella está al lado del vaso = The bottle is next to the glass

Because a + el contracts to al in Spanish.

So:

• a + el lado → al lado

This is a standard contraction, just like:

• de + el → del

For example:

• al lado
• del vaso

You normally must make this contraction when a is followed by el.

Normally, Spanish often says:

• al lado de + noun/pronoun
• next to + something

For example:

• al lado del vaso
• al lado de la mesa

But in your sentence, the thing being referred to is already clear from context, so de + noun/pronoun is left out.

So:

• la botella al lado

is a shortened way of saying something like:

• la botella al lado del vaso
• la botella al lado de él
• la botella al lado de eso

The exact reference depends on the situation.

Because Spanish nouns have grammatical gender.

• el vaso = masculine
• el posavasos = masculine
• la botella = feminine

The article must match the noun:

• masculine singular → el
• feminine singular → la

This is a core part of Spanish grammar, and it does not always match natural gender. Objects also have grammatical gender, so you usually need to learn each noun together with its article.

Because posavasos is one of those Spanish nouns that look plural but can be singular.

Here:

• el posavasos = the coaster

Its form does not change in the singular:

• singular: el posavasos
• plural: los posavasos

This word originally comes from the idea of something that holds/places glasses:

• posa- from posar
• vasos = glasses

But as a complete noun, posavasos works as a normal singular word when needed.

Yes, often you could.

• sobre el posavasos
• encima del posavasos

Both can mean on the coaster / on top of the coaster.

But there is a slight difference in feel:

• sobre is often a bit more direct and compact
• encima de strongly emphasizes on top of

In this sentence, sobre el posavasos sounds perfectly natural.

Yes, it is in the present tense.

• Pongo = present tense

Depending on context, Spanish present can mean:

• I put (habitual or general)
• I’m putting (right now)

So this sentence could mean either:

• a general description of what I do, or
• something I am doing at this moment

Spanish often relies on context to decide which reading is intended.

Yes. If you want to remove ambiguity, you could say:

• Pongo el vaso sobre el posavasos y la botella al lado del vaso.
• Pongo el vaso sobre el posavasos y pongo la botella al lado.

These versions make the structure clearer by either:

• repeating the verb pongo, or
• saying exactly what al lado refers to

But the original sentence is natural because Spanish often leaves out information that is already understood.