Posuda s pijeskom je blizu grebalice.

Because the preposition s meaning with normally requires the instrumental case in Croatian.

• basic noun: pijesak = sand
• instrumental singular: pijeskom

So s pijeskom literally means with sand.

This is a very common pattern:

• kava s mlijekom = coffee with milk
• čovjek sa šeširom = a man with a hat
• posuda s pijeskom = a container with sand

Because blizu takes the genitive case.

The dictionary form is:

• grebalica = scratching post / scratcher

After blizu, it changes to genitive singular:

• grebalice

So:

• blizu grebalice = near the scratching post

This is similar to other Croatian words that require the genitive after them.

Posuda is in the nominative singular because it is the subject of the sentence.

The sentence structure is:

• Posuda s pijeskom = the container with sand
• je = is
• blizu grebalice = near the scratching post

So posuda is the thing being talked about, which is why it stays in the nominative.

Je is the 3rd person singular present tense of the verb biti = to be.

So here it means is.

• posuda ... je blizu ... = the container ... is near ...

Croatian often uses forms of biti just like English uses am / is / are.

In sentences like this, learners often meet blizu as a word meaning near / close to, and it behaves very much like a preposition-like word because it is followed by the genitive.

So in practice, the important thing to remember is:

• blizu + genitive

Examples:

• blizu kuće = near the house
• blizu škole = near the school
• blizu grebalice = near the scratching post

You do not need to worry too much at first whether a grammar book labels it as an adverb or preposition-like expression; the key point is the case it requires.

Because Croatian has no articles.

English distinguishes:

• a container
• the container

Croatian simply says posuda, and the exact meaning depends on context.

So Posuda s pijeskom je blizu grebalice can mean:

• The container with sand is near the scratching post
• or, depending on context, A container with sand is near a/the scratching post

Usually the situation makes it clear.

Yes. Croatian word order is more flexible than English word order.

The neutral order here is:

• Posuda s pijeskom je blizu grebalice.

But other orders are possible for emphasis, for example:

• Blizu grebalice je posuda s pijeskom.
• Posuda je blizu grebalice, s pijeskom. — less natural in this exact meaning, but Croatian can move parts around depending on focus

Even though the order can change, the original version sounds normal and neutral.

Both s and sa mean with, but sa is usually used for easier pronunciation before certain sounds or consonant clusters.

Here, pijeskom begins with p, so s pijeskom is the normal form.

Compare:

• s prijateljem = with a friend
• sa sestrom = with a sister
• sa školom can appear because s školom is harder to pronounce

So in your sentence, s pijeskom is exactly what you would expect.

Grebalica is a feminine noun.

You can tell from the dictionary form ending in -a, which is very often feminine in Croatian.

That matters because feminine nouns have their own case endings. Here:

• nominative singular: grebalica
• genitive singular: grebalice

That is why after blizu you get grebalice.

It is just a normal noun phrase:

• posuda = container / bowl / tray
• s pijeskom = with sand

So literally it means container with sand.

Depending on context, it might refer to something more specific, but grammatically it is simply a noun followed by a with-phrase describing it.

A native speaker would likely understand it very straightforwardly as a statement about location:

• one object is the container with sand
• the other object is the scratching post
• the sentence says the first one is near the second one

If the context is about pets, the exact real-world interpretation could depend on the situation, but grammatically the sentence is very clear.