El cubo está al lado del fregadero, y dentro guardamos los productos de limpieza.

Not here. Cubo can mean different things depending on context:

• cube in maths or geometry
• bucket
• bin / pail / container in household language

In this sentence, it is almost certainly a household container, not a geometric cube. In Spain, cubo often means a bucket or sometimes a bin, especially in domestic contexts.

Because Spanish uses estar for location.

• El cubo está al lado del fregadero = the container/bin is next to the sink

A quick rule:

• ser = identity, definition, what something is
• estar = location, condition, temporary states

So for where something is physically located, estar is the normal choice.

Al lado de means next to, beside, or at the side of.

It is a fixed expression:

• al lado de + noun

Examples:

• al lado de la puerta = next to the door
• al lado del fregadero = next to the sink

The al is the contraction of a + el.

Because in Spanish, de + el contracts to del.

So:

• de el fregadero → del fregadero

This is not optional; it is the normal form.

There are two very common contractions in Spanish:

• a + el = al
• de + el = del

In Spain, fregadero usually means the kitchen sink.

This is useful because learners often confuse it with:

• lavabo = sink/basin, often in a bathroom, or even the bathroom itself in some contexts
• fregadero = the sink used for washing dishes, typically in the kitchen

So in this sentence, fregadero is specifically the natural word for kitchen sink in Spain.

Dentro can work by itself when the thing it refers to is already clear from context.

Here, dentro means inside, and it clearly refers back to el cubo.

So:

• y dentro guardamos los productos de limpieza = and inside we keep the cleaning products

If you want to be more explicit, you could say:

• y dentro de él guardamos los productos de limpieza
• y dentro del cubo guardamos los productos de limpieza

So both patterns exist:

• dentro = inside
• dentro de + noun/pronoun = inside of + something

Because guardar is a classic false friend.

In many everyday contexts, guardar means:

• to keep
• to store
• to put away

So here:

• guardamos los productos de limpieza = we keep/store the cleaning products

It does not usually mean to guard in the English sense of protecting something with security. For that idea, Spanish often uses words like vigilar or proteger, depending on context.

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

• guardamos = we keep/store

The ending -amos tells you it is we.

You could say:

• y nosotros guardamos los productos de limpieza

But that would usually sound more emphatic, as if you were stressing we.

Because los makes it sound like a specific, known set of cleaning products.

• guardamos los productos de limpieza = we store the cleaning products
• guardamos productos de limpieza = we store cleaning products / some cleaning products

Spanish uses the definite article more often than English does. In this sentence, los sounds natural because it refers to the cleaning products that belong there or are normally kept there.

Usually, no. In standard Spanish, you normally do not put a comma before y when joining two closely connected parts of a sentence.

So the more typical version would be:

• El cubo está al lado del fregadero y dentro guardamos los productos de limpieza.

The comma in your sentence is not impossible, but it feels more like a stylistic pause than a basic rule.

Yes. Junto a is a very natural alternative.

• al lado del fregadero = next to the sink
• junto al fregadero = next to the sink / right by the sink

Both are good.
A few useful comparisons:

• junto a = right next to
• al lado de = next to / beside
• cerca de = near

One thing to avoid is a lado de, which is not correct.

Yes, but the original order is very natural.

Spanish often moves adverbs like dentro to the front for emphasis or smoother flow. Here it highlights the place first: inside.

So:

• y dentro guardamos los productos de limpieza = and inside we keep the cleaning products

Other possible versions are:

• y dentro de él guardamos los productos de limpieza
• y guardamos los productos de limpieza dentro

The original version sounds quite natural because dentro clearly refers back to el cubo and gives the sentence a neat rhythm.