Learn Hardcore Croatian: Večeras sam slobodan, pa možemo pisati pisma ili gledati film. - Tonight I am free, so we can write letters or watch a movie.

Večeras sam slobodan… – neutral, common, focus on tonight.
Ja sam večeras slobodan… – emphasizes I (e.g. I am free tonight, not someone else).
Slobodan sam večeras… – emphasizes free, often as a contrast (e.g. I’m not free other days).

So yes, Ja sam večeras slobodan is grammatical; you just change the nuance a bit.

Why is it slobodan and not something like slobodna or slobodno?

The adjective must agree with the subject in gender, number, and case.

ja (I, when the speaker is male) → masculine singular
so the adjective is slobodan (masculine singular, nominative)

If a woman is speaking, she would say:

Večeras sam slobodna. (feminine singular)

Neuter slobodno would be used with neuter nouns or in some impersonal expressions, e.g.:

Večeras je slobodno. (It is free / available tonight – e.g. a seat, a room)

Why is the pronoun ja (I) missing? Shouldn’t it be Ja sam slobodan?

Croatian usually omits subject pronouns because the verb ending already shows the person:

sam = 1st person singular of biti (to be) → “I am”

So Večeras sam slobodan is the default, natural version.

You can add Ja:

Ja sam večeras slobodan.

…but that usually adds emphasis on the person I, like:

Ja sam večeras slobodan, ali on nije.
(I am free tonight, but he isn’t.)

What exactly does pa mean here, and how is it different from i?

In this sentence, pa is best understood as “so” / “so then” / “and so”:

Večeras sam slobodan, pa možemo…
→ I’m free tonight, *so we can…*

Rough differences:

i = and, just connecting ideas:
Večeras sam slobodan i možemo gledati film.
(Simple addition: I’m free and we can watch a movie.)
pa = and so / and then, often implying a result or next step:
Večeras sam slobodan, pa možemo gledati film.
(Because I’m free, as a result we can watch a movie.)

In spoken language, pa is very common and can sound a bit more conversational.

Why is it možemo pisati and not something like možemo da pišemo or možemo da pišemo pisma?

With modal verbs like moći (can, be able to), Croatian normally uses an infinitive directly:

možemo pisati – we can write
možeš gledati – you can watch
moram raditi – I must work

So the usual structure is:

moći (conjugated) + infinitive

You can sometimes hear možemo da pišemo in some dialects or informal speech, but in standard Croatian možemo pisati is preferred and considered more correct.

Why is it pisati pisma (write letters, plural)? Could it be pisati pismo in the singular?

Both are possible, with a small nuance:

pisati pisma – write letters (in general, some letters, maybe more than one)
pisati pismo – write a (single, specific) letter

So:

Možemo pisati pisma – we can sit down and write (some) letters, maybe to different people.
Možemo pisati pismo – we can write one specific letter, for example an application, a complaint, etc.

Using the plural here feels very natural when you mean the activity of letter‑writing in general.

Which case are pisma and film in, and why do they look like nominative?

Both are direct objects of the verbs pisati and gledati, so they’re in the accusative case.

pismo – neuter singular
Accusative plural of pismo is pisma.
Neuter nominative plural and accusative plural look the same: pisma.
film – masculine singular, inanimate
For inanimate masculine nouns, nominative singular and accusative singular are identical: film.

So:

Pisati pisma – pismA = accusative plural
Gledati film – film = accusative singular

Why is there no word for “a / the” before film? How do you know if it’s a film or the film?

Croatian has no articles (a / an / the). Nouns usually appear without any article-like word:

gledati film – watch a film / the film
čitati knjigu – read a book / the book

Whether English should use a or the depends on context, not on a specific Croatian word:

If both speakers know which film: the film
If it’s just any film: a film

Croatian doesn’t mark that difference with a special word.

Could we also say gledati neki film or gledati jedan film? What is the difference from gledati film?

Yes, you can:

gledati film – watch a film / the film (neutral)
gledati neki film – watch some film, a certain film, often vague or unimportant which one
gledati jedan film – literally watch one film; can stress one as a number, or feel like a film in a slightly more specific way

In everyday speech, gledati film is the most natural if you don’t need extra nuance.

Why are pisati and gledati in the infinitive? Could you use forms like napisati or pogledati instead?

pisati and gledati are imperfective infinitives, focusing on the ongoing activity:

pisati – to write (activity, process)
gledati – to watch (activity, process)

You can use perfective verbs:

napisati – to write (finish writing)
pogledati – to watch (finish watching)

These change the meaning:

Možemo pisati pisma – we can (sit and) write letters, focusing on the activity.
Možemo napisati pisma – we can get the letters written, get them finished.

Similarly:

Možemo gledati film – we can watch a film (the activity).
Možemo pogledati film – we can watch (and finish) a film, see it through to the end.

In your original sentence, the imperfective fits the idea of possible activities for the evening.

If a woman is speaking, exactly what changes in the sentence?

Only the adjective changes to agree with the speaker’s gender:

Male speaker: Večeras sam *slobodan, pa možemo pisati pisma ili gledati film.*
Female speaker: Večeras sam *slobodna, pa možemo pisati pisma ili gledati film.*

Everything else stays the same.

Why is the present tense (sam slobodan, možemo) used for a future meaning (tonight)? Could we say Večeras ću biti slobodan instead?

Croatian often uses the present tense for planned or scheduled future actions, especially with time words like večeras, sutra, sljedeći tjedan, etc.

So:

Večeras sam slobodan
= I’m free tonight (natural, common)

You can say:

Večeras ću biti slobodan. – I will be free tonight.

This sounds a bit more formal or as if you’re predicting or promising your future state, rather than just stating a plan or arrangement. In everyday speech, Večeras sam slobodan is more usual.

What exactly does Večeras mean, and how is it different from noćas?

večeras = this evening / tonight, typically the evening period.
noćas = tonight, but more focused on the night (later hours, during the night).

In many contexts they can overlap, but:

Večeras sam slobodan. – I’m free this evening.
Noćas neću spavati. – I won’t sleep tonight / during the night.

In your sentence, Večeras is the natural choice because writing letters or watching a movie is more of an evening activity than a middle‑of‑the‑night action.

Why is there a comma before pa? Is it required?

In standard Croatian, you normally put a comma before conjunctions like pa, i, ali, nego when they connect whole clauses (each with its own verb):

Večeras sam slobodan, → first clause (verb: sam)
pa možemo pisati… → second clause (verb: možemo)

Since pa connects two clauses, the comma is correct and expected here.