Using the correct preposition will prevent you from being misunderstood by other people so it’s important to know how to correctly use them in a sentence. 

“u” and “na” is used when:

1. you are going “to” a place
2. something/someone is “in/on” a place

When you use “u”, it’s inside something

When you use “na”, it’s on something

For example:

The cat is in the box.

Mačka je u kutiji. 

The cat is on the box.

Mačka je na kutiji.

I am in the store.

Ja sam u prodavnici.

I am on the store.

Ja sam na prodavnici.

Now, the use of “iz” and “sa” as from. 

“iz” and “sa” is used when:

1. you came out “from” a place
2. something/someone was “from” a place

Remember this

The partner of “u” is “iz” 

and

The partner of “na” is “sa”

For example:

The cat went out of the box.

Mačka je izašla iz kutije. 

The cat jumped out from the box.

Mačka je skočila sa kutije.

I went out from the store.

Izašla sam iz prodavnice.

I jumped from the store.

Skočio sam sa prodavnice.

Another example:

u - iz

I entered the tunnel.

Ušla sam u tunel.

I went out of the tunnel.

Izašla sam iz tunela.

na - sa

I am on the mountain.

Ja sam na planini.

I came down from the mountain.

Sišla sam sa planine.