U trgovini ću probati druge traperice, jer ove ne stoje dobro bez remena.

This is a very common Croatian word-order pattern.

Ću is a clitic form of htjeti used to make the future tense, and clitics usually like to stand in the second position of the clause.

So in:

U trgovini ću probati druge traperice

the first unit is U trgovini, and then the clitic ću comes right after it.

You could also hear:

Probat ću druge traperice u trgovini.

That is also natural. In that version, the infinitive probati loses its final -i before ću:

• probati ću → not standard
• probat ću → standard

So both patterns are normal:

• U trgovini ću probati...
• Probat ću u trgovini...

Trgovini is in the locative singular.

The preposition u can take:

• locative when it means in/inside a place
• accusative when it means into/to a place

Here, U trgovini means in the shop/store, so it uses the locative.

Compare:

• u trgovini = in the shop (location)
• u trgovinu = into the shop (movement)

Traperice is grammatically plural. It works like English jeans: even when you mean one item, the word is plural.

So Croatian normally says:

• traperice su nove = the jeans are new
• ove traperice = these jeans

If you want to be more explicit, you can say:

• jedne traperice = one pair of jeans
• par traperica = a pair of jeans

Because traperice is treated as feminine plural, words agreeing with it also appear in feminine plural forms.

Because these words must agree with traperice in gender, number, and case.

Since traperice is:

• feminine
• plural

you get:

• druge traperice = other/different jeans
• ove = these

In the first clause, druge traperice is the object of probati, so it is in the accusative plural.

In the second clause, ove means these ones / these jeans and acts as the subject, so it is in the nominative plural.

For this noun, the visible forms happen to look the same:

• nominative plural: traperice
• accusative plural: traperice

So the sentence looks very neat, but the function is different in each clause.

Here it means try on.

In a clothing context, probati very commonly means:

• to try on clothes
• to try something for fit

So probati traperice naturally means to try on jeans.

You may also hear:

• isprobati

That can also mean try on / test out, and sometimes sounds a bit more explicit or complete. But in a shop, probati is completely normal and idiomatic.

Yes, literally stajati / stojati is related to standing, but with clothes dobro stajati is an idiomatic expression.

With clothing, dobro stajati means something like:

• to look good on someone
• to fit well
• to sit right

So:

ove ne stoje dobro bez remena

means that these jeans do not sit/look right without a belt.

It does not necessarily mean the size is completely wrong; it can mean they hang badly, slip down, or just do not look right unless worn with a belt.

The preposition bez always takes the genitive.

So:

• nominative: remen = belt
• genitive: remena

That is why the sentence says:

bez remena = without a belt

This is a very useful pattern to remember:

• bez šećera = without sugar
• bez vode = without water
• bez kaputa = without a coat
• bez remena = without a belt

Because the speaker is giving a reason that is true now.

The sentence means something like:

• I’ll try other jeans in the store, because these ones don’t fit/look right without a belt.

The trying-on will happen in the future, but the problem with these jeans is a current fact, so Croatian uses the present tense:

• ne stoje dobro = they don’t sit/look right

This is perfectly natural. English does the same thing:

• I’ll get another pair, because these don’t fit right.

Yes. Jer is a very common, natural word meaning because.

So:

• ..., jer ove ne stoje dobro bez remena.

is completely normal.

You could also use:

• zato što

For example:

• U trgovini ću probati druge traperice, zato što ove ne stoje dobro bez remena.

Both mean because, but jer is often shorter and a bit more straightforward in everyday speech.