Poslije kiše tobogan je mokar, pa kantica i lopatica ostaju pokraj pješčanika.

Because poslije normally takes the genitive case. The noun kiša becomes kiše in the genitive singular.

• kiša = rain
• poslije kiše = after the rain / after rain

This is a very common pattern in Croatian.

Yes. Poslije kiše and nakon kiše both mean after the rain and both take the genitive.

A rough difference:

• poslije is very common in everyday speech
• nakon is also common, and can sound slightly more neutral or formal in some contexts

In this sentence, either one works.

Because je is an unstressed form of biti and usually does not stand at the beginning of its clause by itself. In the basic clause tobogan je mokar, it naturally comes after tobogan.

Croatian word order is flexible, so Poslije kiše je tobogan mokar is also possible, but Poslije kiše tobogan je mokar is very natural.

Here mokar is a predicate adjective, used after je.

• Tobogan je mokar. = The slide is wet.

When an adjective comes directly before a noun, you often see:

• mokri tobogan = a/the wet slide

So the short answer is:

• after je: mokar
• before the noun: often mokri

Yes. In Croatian, tobogan usually means a slide, especially a playground slide or water slide.

So this is a classic false-friend warning for English speakers:

• English toboggan = a type of sled
• Croatian tobogan = slide

Here pa means something like so, and so, or therefore.

It links the two parts of the sentence:

• the slide is wet
• so the bucket and shovel stay by the sandbox

It is a very common conversational connector in Croatian.

Because the subject is kantica i lopatica — two things joined by i.

So Croatian uses 3rd person plural:

• ostaju = they stay / remain

If there were only one item, you would use singular:

• kantica ostaje
• lopatica ostaje

Ostaju is the normal present-tense form here and means stay / remain.

• ostaju = imperfective, present, ongoing/general meaning
• ostanu = from a perfective verb, and usually does not fit as well in this kind of simple descriptive sentence

So in a sentence describing the situation in general, ostaju is the natural choice.

Because pokraj takes the genitive case. The noun pješčanik becomes pješčanika in the genitive singular.

• pješčanik = sandbox
• pokraj pješčanika = beside / next to the sandbox

So this is similar to poslije kiše: a preposition is forcing the next noun into the genitive.

Yes. Pokraj pješčanika and pored pješčanika are both natural and both mean beside / next to the sandbox.

You may also hear kraj pješčanika. All three are common, though exact preference can depend on region and style.

Yes, they are often understood that way.

• kantica = little bucket / bucket, especially a child’s sand bucket
• lopatica = little shovel / scoop / spade, especially a toy one

The -ica ending often has a diminutive feel, but in everyday speech these are also just the normal words for children’s sand toys.

Because Croatian does not have articles like English the and a.

So:

• tobogan can mean a slide or the slide
• kantica can mean a bucket or the bucket

The exact meaning comes from context. That is completely normal in Croatian.

Croatian word order is fairly flexible. The given version is natural, but other versions are possible if you want to shift emphasis.

For example, you could also hear something like:

• Poslije kiše je tobogan mokar, pa kantica i lopatica ostaju pokraj pješčanika.

The core meaning stays the same, but the rhythm or focus changes a little. English is usually stricter about word order than Croatian.