Learn Hardcore Croatian: I ja sam uzbuđen, jer ćemo preksutra zajedno ići na stadion. - I am also excited because the day after tomorrow we will go to the stadium together.

"Ja sam uzbuđen." – a simple statement about yourself.
"I ja sam uzbuđen." – you are adding yourself to someone else who is already excited (or to a list of people). It has the nuance “me too”.

You could express the same idea in other ways:

"Ja sam isto uzbuđen." – I am also excited.
"I ja sam jako uzbuđen." – I too am very excited.

So "I" here doesn’t just mean “and”, it functions like “also / too” and emphasizes that you share the same feeling as someone else.

If the speaker is female, does "uzbuđen" change? How does this agreement work?

Yes, it changes to agree with the gender and number of the subject.

The form in the sentence is masculine singular:

(ja) sam uzbuđen – I am excited (speaker = man/boy).

Other common forms:

Singular:

uzbuđen – masculine (on je uzbuđen)
uzbuđena – feminine (ona je uzbuđena)
uzbuđeno – neuter (dijete je uzbuđeno – the child is excited)

Plural:

uzbuđeni – masculine or mixed group (oni su uzbuđeni)
uzbuđene – feminine group (one su uzbuđene)
uzbuđena – neuter plural (rare in everyday speech, e.g. djeca su uzbuđena)

So a female speaker would say:

"I ja sam uzbuđena, jer ćemo preksutra zajedno ići na stadion."

Can I drop "sam" and just say "I ja uzbuđen"?

Not in a normal, full sentence. You need "sam" (a form of biti = “to be”) for the present tense:

"Ja sam uzbuđen." – correct
"Ja uzbuđen." – sounds incomplete/wrong on its own.

You can sometimes drop it in very elliptical or poetic speech, usually when the verb is understood from context, but that’s not standard neutral Croatian.

For everyday correct Croatian, keep the auxiliary:

"I ja sam uzbuđen." – I am excited too.
"I ja sam bila uzbuđena." – I (female) was excited too.

Why is there a comma before "jer"? Is it obligatory?

Yes, in standard Croatian you generally put a comma before "jer" when it introduces a clause meaning “because”.

Your sentence has two clauses:

I ja sam uzbuđen,
jer ćemo preksutra zajedno ići na stadion.

They are joined by "jer" (because), so a comma is used:

"I ja sam uzbuđen, jer ćemo preksutra zajedno ići na stadion."

Leaving the comma out is usually considered a punctuation mistake in standard writing. In informal online writing people sometimes skip it, but formally it should be there.

Why is "jer" used here and not "zato što" or "pošto"? Do they all mean “because”?

They are all used to express reason, but with slightly different feel:

"jer" – very common, neutral “because”, often used to answer "Zašto?" (Why?).
- I ja sam uzbuđen, jer ćemo preksutra zajedno ići na stadion.
"zato što" – also “because”, often a bit more explicit/stronger; it literally starts with "zato" (for that reason) plus "što" (that).
- I ja sam uzbuđen, zato što ćemo preksutra zajedno ići na stadion.
"pošto" – in Croatia, as “because”, it is more colloquial/regional and in some contexts ambiguous (also means “after” in time). Many teachers advise learners to prefer "jer" or "zato što".
- I ja sam uzbuđen, pošto ćemo preksutra zajedno ići na stadion. (sounds informal/colloquial)
"budući da" – more formal, like “since / given that”.
- I ja sam uzbuđen, budući da ćemo preksutra zajedno ići na stadion.

In your sentence, "jer" is the most natural, neutral choice.

How does "ćemo ... ići" work? Why do we need both "ćemo" and "ići"?

This is the future tense (Futur I) in Croatian, formed with:

the present tense of htjeti (as a clitic: ću, ćeš, će, ćemo, ćete, će)
plus the infinitive of the main verb.

For ići (to go):

ja ću ići – I will go
ti ćeš ići – you will go
on/ona/ono će ići – he/she/it will go
mi ćemo ići – we will go
vi ćete ići – you (pl) will go
oni/one/ona će ići – they will go

So "ćemo" tells you the person and number (we), and "ići" is the main meaning (“to go”). Together: "ćemo ići" = "we will go".

In your sentence:

"jer ćemo preksutra zajedno ići na stadion"
= because we will go to the stadium together the day after tomorrow.

Could I say "ići ćemo" instead of "ćemo ići"?

Yes, both are possible; the difference is mainly in word order and emphasis.

Both of these are grammatically correct:

"Prekosutra ćemo ići na stadion."
"Prekosutra ići ćemo na stadion."

However, in everyday speech:

Putting the clitic ("ćemo") in second position is more common:
- "Prekosutra ćemo ići na stadion." (very natural)
Starting with the infinitive ("Ići ćemo...") is also correct but can sound a bit more formal, emphatic, or stylistic:
- "Ići ćemo prekosutra na stadion."

In your sentence, because the clause begins with "jer", the clitic must come right after "jer":

"jer ćemo preksutra zajedno ići na stadion" – correct
"jer ići ćemo preksutra zajedno na stadion" – sounds odd in standard Croatian.

Why is "ćemo" immediately after "jer"? Can I say "jer preksutra ćemo zajedno ići..."?

You normally cannot move "ćemo" away from the second position like that.

In Croatian, short unstressed words like ću, ćeš, će, ćemo, ćete, će (and others such as sam, si, je, mi, ti, se, ga, joj...) are clitics. A key rule is:

In a clause, clitics usually go in the second position (after the first stressed word or phrase).

In your clause:

First word: "jer"
Second “slot”: "ćemo" → "jer ćemo preksutra..."

This is correct.

Now compare:

"Prekosutra ćemo zajedno ići na stadion."
- First word: Prekosutra
- Second position: ćemo – correct.
"Jer preksutra ćemo zajedno ići na stadion."
- Here "ćemo" is in third position (after jer preksutra), so it sounds wrong in standard Croatian.

So the correct options are:

"jer ćemo prekosutra zajedno ići na stadion"
"Prekosutra ćemo zajedno ići na stadion"

but not "jer prekosutra ćemo...".

Is "preksutra" correct, and what exactly does it mean?

It means “the day after tomorrow”.

The most standard written form in Croatian is:

"prekosutra" (with -ko-):
- Prekosutra idemo na stadion. – We’re going to the stadium the day after tomorrow.

You will hear both "prekosutra" and "preksutra" in speech, depending on region and speaker. For learners, it’s safer to stick to "prekosutra" in writing and formal contexts.

Related time words:

jučer – yesterday
prekjučer – the day before yesterday
sutra – tomorrow
prekosutra – the day after tomorrow

Where can "zajedno" go? Is "zajedno ići na stadion" the only correct order?

"Zajedno" is an adverb meaning “together”, and it is quite flexible in word order. All of these are possible:

"Prekosutra ćemo zajedno ići na stadion."
"Prekosutra ćemo ići zajedno na stadion."
"Zajedno ćemo prekosutra ići na stadion."

The differences are mainly in emphasis:

"zajedno ćemo..." – emphasizes “together” (we, as a group, will go).
"ćemo zajedno ići" – neutral, very natural word order.
"ćemo ići zajedno" – slightly more focus on the act of going, then adds that it is together.

Your version, "preksutra zajedno ići na stadion", is fine inside the longer clause:

"...jer ćemo preksutra zajedno ići na stadion."

It sounds completely natural.

Why is it "na stadion" and not "u stadion"? What’s the difference?

The preposition depends on how the place is conceptualized.

"na" – literally “on”, but very often used as “to / at” for open or surface-like places, events, and institutions:
- na stadion – to the stadium (as a venue)
- na koncert – to a concert
- na utakmicu – to a match
- na posao – to work
"u" – literally “in / into”, used more for enclosed spaces and interiors:
- u kuću – into the house
- u školu – into the school
- u kino – into the cinema

For stadion, the natural expression when you mean going there for an event is:

"ići na stadion" – to go to the stadium.

"u stadion" would sound like going inside the structure itself (physically into the body of the stadium), which is not how people usually phrase it when talking about attending a match or event.

Why is it "na stadion" and not "na stadionu"?

This is a case difference:

"na stadion" – accusative: used with movement towards a place (to the stadium).
"na stadionu" – locative: used for being at a place (at/on the stadium).

Compare:

Idemo na stadion. – We are going to the stadium. (motion → accusative)
Mi smo na stadionu. – We are at the stadium. (location → locative)

In your sentence you are talking about going there, so "na stadion" (accusative) is correct:

"...jer ćemo preksutra zajedno ići na stadion." – because we will go to the stadium together the day after tomorrow.