Learn Hardcore Croatian: Nakon dugog dana najveća mi je sreća kad legnem u krevet i osjetim pravi odmor. - After a long day my greatest happiness is when I lie down in bed and feel real rest.

dan (day) in nominative becomes dana in genitive.
The adjective dug (long) has to match the noun in case, so in masculine genitive singular it becomes dugog.

So the pattern is:

nakon + genitive → nakon dugog dana, nakon posla, nakon napornog rada.

Using nakon dugi dan would be ungrammatical because dugi dan is nominative, not genitive.

What is the difference between nakon and poslije?

Both can mean after, but there are small differences:

nakon is only a preposition and always requires genitive:
- nakon dana, nakon škole, nakon ručka
poslije can be:
- a preposition: poslije dana, poslije škole
- an adverb on its own: Vidimo se poslije. (See you later/afterwards.)

In everyday speech, poslije is extremely common; nakon can sound a bit more neutral/formal or written. In this sentence, you could also say:

Poslije dugog dana…

with no change in meaning.

What does mi mean in najveća mi je sreća, and is it necessary?

mi is the dative form of ja (I), so literally “to me / for me”.

Here it is a so‑called dative of interest or ethical dative. It adds a personal nuance:

Najveća mi je sreća… ≈ The greatest happiness *for me personally is…*

Is it necessary?

Grammatically, you can omit it: Najveća je sreća kad legnem… – still correct, but sounds more impersonal or general.
With mi, it feels warmer and more subjective, stressing that this is my own feeling.

You could also say:

Meni je najveća sreća kad… (more explicit meni, not clitic mi)

Why is the word order najveća mi je sreća, and can I change it?

Croatian has relatively flexible word order, but clitic pronouns like mi and the verb je like to stand in second position in the clause.

In najveća mi je sreća:

najveća is the first stressed word in that clause,
then the clitics come: mi je,
then the noun: sreća.

Other natural variants:

Najveća sreća mi je kad legnem…
Meni je najveća sreća kad legnem…

All of these are grammatical. The given version Najveća mi je sreća… slightly emphasizes najveća (“the greatest happiness for me…”).

Is there any difference between kad and kada in kad legnem u krevet?

Meaning-wise, no. Both mean when.

kad – shorter, more colloquial, very common in speech and normal in writing.
kada – a bit more formal or emphatic, often used in written or more careful style.

In this sentence you can freely switch:

…kad legnem u krevet…
…kada legnem u krevet…

No change in meaning.

Why is it legnem and not something like ležim or ležem?

legnem is the present tense of the perfective verb leći (to lie down, to get into a lying position).

leći (pf.) → legnem, legneš, legne… – the act of lying down (a single completed action).
ležati (impf.) → ležim, ležiš… – the state of lying.

In this sentence, we are talking about the moment:

kad legnem u krevet – when I lie down in bed (the moment I get into bed).

If you said kad ležim u krevetu, it would focus on the ongoing state of already lying in bed, not the action of lying down. The version with legnem fits better with a single, satisfying moment at the end of the day.

(Forms like ležem are dialectal/colloquial; standard language distinguishes leći/ležati as above.)

Why is it u krevet and not u krevetu?

Because u + accusative expresses movement into something, while u + locative expresses location in something.

u krevet (accusative) – into the bed (direction / goal)
- Legnem u krevet. – I lie down into bed.
u krevetu (locative) – in the bed (location)
- Ležim u krevetu. – I’m lying in bed.

In this sentence you’re describing going to bed (movement), so u krevet is correct.

What is the difference between osjetim and osjećam?

They come from two different aspects of the verb:

osjetiti (pf.) → osjetim, osjetiš… – to feel / sense at a particular moment, a completed act of starting to feel something.
osjećati (impf.) → osjećam, osjećaš… – to feel continuously or repeatedly.

In the sentence:

…i osjetim pravi odmor. – “and I (suddenly) feel real rest” – the moment that feeling appears.

If you said i osjećam pravi odmor, it would describe more the ongoing state while you are resting, not the onset of that feeling. The perfective osjetim matches the idea of that first satisfying feeling when you lie down.

Why are both verbs in the present tense when this sounds like a repeated / typical situation?

Croatian uses the present tense for:

General truths / habits / repeated situations
- Nakon dugog dana… najveća mi je sreća kad legnem…
  ≈ “After a long day, my greatest happiness is (always) when I lie down…”
Future time in time clauses with kad
- Nazovi me kad legneš u krevet. – Call me when you (will) lie down in bed.

So the present legnem and osjetim here express a habitual, typical sequence: whenever I have a long day, this is what happens.

Why is there no comma before kad in this sentence?

In Croatian, a comma is not usually placed between a verb and a subordinate clause that functions as its direct complement (something like a subject or object).

Here:

Najveća mi je sreća – main clause
kad legnem u krevet i osjetim pravi odmor – clause that explains what that greatest happiness is, almost like the predicate complement.

Because this kad‑clause is tightly bound to je (“is”), there is typically no comma:

Najveća mi je sreća kad legnem u krevet…

If you put the kad‑clause first, you do use a comma:

Kad legnem u krevet i osjetim pravi odmor, najveća mi je sreća.

What nuance does pravi odmor have compared to just odmor?

pravi means real / genuine / proper.

odmor – rest, break.
pravi odmor – real rest, proper rest, the kind of rest that really counts.

Adding pravi emphasizes the quality and depth of the rest: not just sitting down for five minutes, but the full, satisfying rest you get when you finally lie in bed.

Why is it najveća and not najveći or najveće?

najveća is the feminine singular form of the superlative adjective najveći (the biggest / greatest).

Adjectives must agree with the noun in:

gender, number, and case.

Here:

sreća (happiness) is feminine singular nominative.
So the adjective must also be feminine singular nominative: najveća.

Other forms would go with different noun genders:

najveći problem (masc.)
najveća sreća (fem.)
najveće zadovoljstvo (neut.)

That’s why najveća mi je sreća is the correct form.