Objasni mi čemu služi ova karta ako je staza zatvorena zbog magle.

Yes. Objasni is the imperative singular form of objasniti, so it means explain! when speaking to one person.

A useful extra point is aspect:

• objasniti = perfective, focusing on a completed explanation
• objašnjavati = imperfective, focusing on the process or repeated explaining

So Objasni mi... sounds like Explain it to me with the idea of giving a complete explanation.

Mi means to me.

It is the unstressed dative clitic form of ja. In this sentence:

• Objasni mi = Explain to me

Croatian very often uses short clitic forms like mi, ti, mu, joj instead of the full stressed forms.

Compare:

• Objasni mi. = neutral, normal
• Objasni meni. = more emphatic, like Explain to me specifically

Because mi is a clitic, and Croatian clitics usually go in or near the second position in the clause.

So:

• Objasni mi... = natural
• Mi objasni... = not natural in standard usage

This is one of the most important word-order habits to learn in Croatian: short words like mi, ti, se, ga, je often cannot just go anywhere.

Literally, čemu služi means something like to what does it serve or what is it used for.

In natural English, you would usually translate it as:

• what is it for
• what is it used for

So:

• čemu služi ova karta = what is this map for / what is this map used for

Because the verb služiti in the sense of to serve / to be used for takes the dative.

So Croatian asks:

• Čemu služi? = What is it for?

Here čemu is the dative form of što.

This is something you mostly just learn as a pattern:

• služiti čemu = to serve for / be used for something

Because ova karta is the subject of služi, so it stays in the nominative.

Breakdown:

• ova = feminine singular nominative of this
• karta = feminine singular nominative

So:

• ova karta služi = this map serves / is used

If it were a direct object, then you would expect the accusative:

• Vidim ovu kartu. = I see this map.

In this context, probably yes.

Since the sentence also mentions staza (trail/path), karta is most naturally understood as map.

But karta is a broad Croatian word and can also mean:

• ticket
• card
• playing card
• map

So learners should remember that context decides the meaning.

Because zatvorena agrees with staza.

• staza = feminine singular
• zatvorena = feminine singular form of closed

So:

• staza je zatvorena = the trail is closed

This is a very common Croatian pattern: the adjective or participle agrees with the noun in gender and number.

Compare:

• put je zatvoren = the road is closed
• staza je zatvorena = the trail is closed
• vrata su zatvorena = the door/doors are closed

Because je is the present-tense form of biti (to be), and Croatian uses it here just like English uses is.

So:

• staza je zatvorena = the trail is closed

This is not a separate action like someone closes the trail. It describes a state:

• zatvorena = closed
• je zatvorena = is closed

Because zbog means because of / due to, and it requires the genitive case.

The noun is:

• magla = fog

Its genitive singular form is:

• magle

So:

• zbog magle = because of the fog / due to fog

This is another useful pattern to memorize:

• zbog + genitive

For example:

• zbog kiše = because of the rain
• zbog snijega = because of the snow

The word order is fairly natural, but Croatian word order is somewhat flexible.

This sentence is neutral and clear:

• Objasni mi čemu služi ova karta ako je staza zatvorena zbog magle.

You can move parts around for emphasis, but not completely freely, especially because of clitics like mi.

For example, you might also hear or write:

• Objasni mi, ako je staza zatvorena zbog magle, čemu služi ova karta.

In careful writing, many teachers would also expect a comma before the ako clause.

So the main things to remember are:

• the general meaning stays the same
• mi still wants its clitic position
• word order can shift for focus or style