Learn Hardcore Croatian: Ostajem sam kod kuće večeras, jer je hladno. - I am staying alone at home tonight because it is cold.

Ostajem sam/sama. = I’m staying alone.
The short form of to be (also sam) is a clitic used with predicates or as an auxiliary in past tenses: Ja sam umoran. (I am tired.) / Ostao/ostala sam. (I stayed.) Context and grammar tell you which sam it is.

If I’m female, do I still say sam?

No. As an adjective, sam agrees with gender and number:

masculine: sam (Ja ostajem sam.)
feminine: sama (Ja ostajem sama.)
plural masc. mixed: sami (Mi ostajemo sami.)
plural fem.: same (Mi ostajemo same.)

Why is there no subject pronoun ja?

Croatian usually drops subject pronouns because the verb ending shows the person. Ostajem already means “I am staying.” You can add Ja for emphasis or contrast: Ja ostajem, a ti idi.

What exactly is ostajem in terms of tense/aspect, and why not ostanem?

Ostajem is present tense of the imperfective verb ostajati (“to be staying/remain” as an ongoing or habitual action).
Ostanem is present of the perfective ostati and is typically used for future/hypothetical contexts (e.g., Ako ostanem... = If I stay...). For a decided plan tonight, the imperfective present (ostajem) is natural.

How would I say this in the future tense?

Use the future with ću and the infinitive without final -i:

Ostat ću kod kuće večeras. (Future 1; the clitic ću goes to second position.) You can also put a different word first and then ću:
Večeras ću ostati kod kuće.

Why is it kod kuće and not u kući?

kod kuće = at home (idiomatic “at someone’s place/home,” here your own).
u kući = in the house (physically inside a house).
Motion to home is kući (without a preposition): Idem kući.

What case is kuće after kod, and why?

Kod governs the genitive. Kuća (house/home) in the genitive singular is kuće, so kod kuće = “at home.”

Can I say doma instead of kod kuće?

Yes. Doma is a common adverb meaning “at home.” It’s widely used and perfectly fine:

Ostajem doma večeras.

Can I move večeras elsewhere in the sentence?

Yes. Word order is flexible for emphasis:

Večeras ostajem sam kod kuće, jer je hladno. (focus on when)
Ostajem sam večeras kod kuće, jer je hladno.
Ostajem sam kod kuće večeras, jer je hladno. All are natural.

Why is there a comma before jer?

Because jer introduces a subordinate clause of cause (“because…”). In standard Croatian, you put a comma before jer:

..., jer je hladno.

Where does je go in jer je hladno, and can I say jer hladno je?

Je is a clitic and prefers second position in its clause. After jer, it comes immediately: jer je hladno. Saying jer hladno je is not standard.

Why is there no “it” in je hladno?

Croatian doesn’t use a dummy subject like English “it.” Weather expressions use a neuter predicate with biti:

Hladno je. / Je hladno. (in main clauses you’ll typically hear Hladno je.)

Can I start the sentence with Jer je hladno, ...?

In speech and informal writing, yes: Jer je hladno, ostajem... In more formal writing, many prefer Zato što je hladno, ... or they keep the main clause first: Ostajem..., jer je hladno.

Is jer interchangeable with zato što?

Often, yes:

Ostajem..., jer je hladno.
Ostajem..., zato što je hladno. Stylistically, zato što can feel a bit more explicit/neutral in formal text. You can also use nouns: Zbog hladnoće ostajem kod kuće. (“Because of the cold...”)

How do I negate this?

Place ne directly before the verb:

Večeras ne ostajem (sam) kod kuće, jer nije hladno. Note the negated nije in the reason clause.

Is there potential ambiguity between sam = “alone” and sam = “I am”? How is it resolved?

Yes, they’re homographs. Context and grammar resolve it. Dictionaries sometimes mark “alone” with an accent (sâm) to disambiguate, but accents are rarely written in everyday text. In your sentence, after a full verb (ostajem), sam must be the adjective “alone.”

What’s the difference between večeras and noćas?

večeras = tonight, in the evening hours.
noćas = tonight, during the night (late night/overnight). Choose based on when you mean.

Any pronunciation tips for č and ć in večeras and kuće?

č (in večeras) is like the “ch” in English “church,” crisp and hard.
ć (in kuće) is a softer, more palatal sound, somewhat like a soft “ty” in “tune” (in many accents). Don’t worry—context and practice will make them feel natural.