Nasuprot prozoru stoji niski stolić, a pokraj njega plavi ormarić.

Because nasuprot normally takes the dative case in Croatian.

• prozor = window
• prozoru = to / opposite the window in the dative singular

So:

• nasuprot prozoru = opposite the window

This is one of those prepositions/adverbs you just have to learn together with its case pattern.

Croatian word order is much more flexible than English word order. Here, the sentence begins with the location:

• Nasuprot prozoru = opposite the window

That sets the scene first, and then tells you what is there:

• stoji niski stolić = a low little table stands / there is a low little table

This is very natural in descriptions of rooms or pictures. A more neutral alternative would be:

• Niski stolić stoji nasuprot prozoru.

That is also correct, but it puts the table first rather than the location.

In Croatian, verbs like stajati / stoji are very commonly used for the position of objects, including furniture.

So Croatian often says things like:

• Na stolu stoji knjiga. = A book is on the table.
• U kutu stoji ormar. = A wardrobe is standing in the corner.

In English, we often use is, but Croatian prefers a more physical verb in many situations. So stoji here sounds natural and idiomatic.

Because Croatian often omits a repeated verb when it is obvious from the context.

The full version would be:

• Nasuprot prozoru stoji niski stolić, a pokraj njega stoji plavi ormarić.

But since stoji is already understood, Croatian can leave it out:

• ..., a pokraj njega plavi ormarić.

This kind of omission is very common and natural.

Both can sometimes be translated as and, but they do not always feel the same.

• i simply adds one thing to another
• a often links two separate pieces of information, sometimes with a slight contrast or shift in focus

Here, a works well because the sentence is presenting one item and then another item beside it:

• opposite the window there is one thing,
• and beside it there is another thing

So a sounds very natural in this descriptive style. It is not a strong contrast like but, just a gentle separation between two parts of the description.

Yes, njega can mean him, but it can also mean it when it refers to a masculine inanimate noun.

Here it refers back to:

• stolić = little table, which is masculine

So:

• pokraj njega = beside it

Croatian does not have a separate masculine form for him versus it in this structure. Context tells you which meaning is intended.

Because pokraj takes the genitive case.

The pronoun on changes by case:

• nominative: on = he / it
• genitive: njega

So:

• pokraj njega = beside him / it

This is similar to other Croatian prepositions that require a specific case. You learn them as combinations:

• nasuprot + dative
• pokraj + genitive

This is a very common question.

In Croatian, adjectives like these often have two forms in the masculine singular nominative:

• a short form: nizak, plav
• a long form: niski, plavi

In standard Croatian, when the adjective comes before a noun like this, the long form is usually used:

• niski stolić
• plavi ormarić

The short form is much more common in the predicate:

• Stolić je nizak. = The little table is low.
• Ormarić je plav. = The little cabinet is blue.

So in this sentence, niski and plavi are exactly what you would expect before the nouns.

The suffix -ić is very often a diminutive ending.

So:

• stol = table
• stolić = little table, small table

and

• ormar = wardrobe / cabinet
• ormarić = little cabinet, small cupboard

Diminutives in Croatian can indicate:

• small size
• affection
• a more informal everyday label for an object

In furniture vocabulary, they often simply mean that the item is smaller.

They are very close in meaning. All of them can mean something like next to / beside / by.

• pokraj
• pored
• kraj

In many everyday sentences, they are interchangeable or nearly so. There can be differences in style, region, and exact nuance, but for a learner, it is safe to understand pokraj njega as beside it / next to it.

Also note that these prepositions commonly take the genitive.

Because Croatian has no articles like English the and a/an.

So a phrase like:

• niski stolić

can mean:

• a low little table
• the low little table

Which one is meant depends on the context.

Croatian usually expresses definiteness through:

• context
• word order
• demonstratives like ovaj (this), taj (that), onaj (that over there)

So if a speaker wanted to be more specific, they could say something like:

• taj niski stolić = that low little table

But in your sentence, no article is needed.