To je prosto nemoguće.

It can work like either that or it, depending on context.

In To je prosto nemoguće, to is a neuter singular demonstrative pronoun. Serbian often uses to to point to:

• a thing: To je moj telefon. = That is my phone.
• a whole situation or idea: To je nemoguće. = That / It is impossible.

So in this sentence, to most naturally refers to some situation, fact, or claim that has just been mentioned.

Je is the 3rd person singular present form of the verb biti (to be).

So:

• to = that / it
• je = is
• prosto nemoguće = simply impossible

Serbian often uses je the same way English uses is:

• To je tačno. = That is correct.
• To je čudno. = That is strange.

Because nemoguće agrees with to, which is neuter singular.

The adjective nemoguć (impossible) changes form for gender:

• masculine: nemoguć
• feminine: nemoguća
• neuter: nemoguće

So:

• Plan je nemoguć. = The plan is impossible.
  (plan is masculine)
• Ideja je nemoguća. = The idea is impossible.
  (ideja is feminine)
• To je nemoguće. = That/It is impossible.
  (to is neuter)

Here it is functioning as an adjective, not an adverb.

It is a predicate adjective after je, describing to:

• To je nemoguće. = That is impossible.

It may look like an adverb form to an English speaker, because many Serbian neuter adjective forms end in -e, but in this sentence it is agreeing with to as a neuter adjective.

Here prosto means simply, just, or plainly.

It adds emphasis:

• To je nemoguće. = That is impossible.
• To je prosto nemoguće. = That is simply impossible.

So prosto strengthens the statement a little and makes it sound more expressive.

Very close, yes.

You will often hear both:

• To je prosto nemoguće.
• To je jednostavno nemoguće.

Both can mean That is simply impossible.

A rough nuance:

• jednostavno often sounds more neutral and standard
• prosto can sound a bit more emotional, emphatic, or conversational

But in many everyday contexts, they are interchangeable.

Yes, Serbian word order is fairly flexible, though some versions sound more natural than others.

Standard and natural:

• To je prosto nemoguće.

Also possible for emphasis:

• Prosto je nemoguće. = It is simply impossible.
• To je nemoguće. = without prosto
• To prosto nije moguće. = That simply isn’t possible.

The original order is very natural. Putting prosto before nemoguće is common because it modifies the adjective.

Yes. That is a very common alternative.

Compare:

• To je nemoguće. = That is impossible.
• To nije moguće. = That is not possible.

The meaning is very similar, but there is a slight difference in feel:

• nemoguće is more direct and compact
• nije moguće can sound a bit more neutral or analytical

You can also combine them with prosto:

• To prosto nije moguće.

It is basically neutral, and it works well in both speech and writing.

The sentence itself is not slang. However:

• To je nemoguće. sounds very neutral
• To je prosto nemoguće. sounds a little more expressive
• To je jednostavno nemoguće. may sound slightly more standard or careful

So the original sentence is perfectly normal and natural in everyday Serbian.

Yes, absolutely.

Prosto je nemoguće is a complete and natural sentence. It means something like:

• It’s simply impossible
• That’s simply impossible

Leaving out to makes the sentence a bit more general or dramatic. Including to points more clearly to a specific thing or situation already being discussed.

No. Its meaning depends on context.

As an adverb, it can mean:

• simply
• plainly
• just

But the related adjective prost / prosta / prosto can also mean:

• simple
• in some contexts, crude, vulgar, or uncultured

So in To je prosto nemoguće, it clearly means simply, not anything negative.