Learn Hardcore Finnish: Jos otan liian pitkän päiväunen, minun on vaikea nukahtaa yöllä. - If I take too long a nap, it is hard for me to fall asleep at night.

Kun can mean when (in time) or whenever.
Using jos emphasizes that taking a long nap is not certain; it’s a condition.
With kun, it would sound more like something that definitely or regularly happens:

Kun otan liian pitkän päiväunen, minun on vaikea nukahtaa yöllä.
= When(ever) I take too long a nap, it’s hard for me to fall asleep at night.

Why is there no minä in jos otan? Can I say jos minä otan?

The personal ending -n on otan already shows the subject is I:

otan = I take
otat = you take
ottaa = he / she takes

So jos otan is fully grammatical and natural.

You can say jos minä otan, but adding minä usually:

adds emphasis (if I take, as opposed to someone else), or
appears in more careful or formal speech.

In everyday speech, Finns often omit subject pronouns when the verb ending makes the subject clear.

Why do both pitkän and päiväunen end in -n? What case is this?

Both pitkän and päiväunen are in the genitive singular, which in this context functions as the total object of otan:

base forms: pitkä päiväuni = a long nap
in the sentence: pitkän päiväunen = a (whole) long nap, object of otan

In Finnish, adjectives agree with the noun in case and number, so:

nominative: pitkä päiväuni
genitive/accusative (total object): pitkän päiväunen

The -n marks that we are talking about one complete, bounded nap that you take.

Why is it päiväunen and not päiväuni or päiväunta?

The base word is päiväuni (day + sleep → nap).

päiväuni = nominative (dictionary form)
päiväunen = genitive singular
päiväunta = partitive singular

Here, otan liian pitkän päiväunen treats the nap as a whole, complete thing you take, so Finnish uses the total object form (genitive/accusative: päiväunen).

If you used päiväunta, it would suggest an indefinite amount of napping, which doesn’t fit as well with a single nap.

What does liian mean, and why is it before pitkän?

Liian means too / excessively.

It modifies the adjective pitkä (long), and together they mean too long:

pitkä päiväuni = a long nap
liian pitkä päiväuni = a too long nap

In Finnish, intensifiers like liian, hyvin, erittäin usually come before the adjective:

liian pitkä = too long
erittäin hyvä = very good

When you inflect the phrase, both the adjective and the noun change, but liian stays the same:

nominative: liian pitkä päiväuni
genitive object: liian pitkän päiväunen

How does the structure minun on vaikea work, and why is minun in the genitive?

Minun on vaikea nukahtaa literally corresponds to (for) me it is difficult to fall asleep.

The pattern is:

[genitive pronoun] + on + [adjective] + [infinitive verb]

Examples:

Minun on vaikea nukahtaa. = It is difficult for me to fall asleep.
Sinun on helppo ymmärtää. = It is easy for you to understand.

Minun is the genitive of minä. In this construction, the experiencer (the person for whom something is easy / difficult / necessary) is usually in the genitive:

Minun on pakko lähteä. = I have to leave.
Hänen on vaikea keskittyä. = It’s hard for him/her to concentrate.

In spoken language, you’ll often hear the shortened forms:

Mun on vaikea nukahtaa.
Sun on helppo tulla.

Can I say minulle on vaikea instead of minun on vaikea?

Not in this structure.

Minun on vaikea nukahtaa is a fixed, very common pattern: genitive pronoun + on + adjective + infinitive.

Minulle on vaikea nukahtaa is ungrammatical.

With minulla / sinulla / hänellä etc. you get different meanings, e.g.:

Minulla on vaikea nimi. = I have a difficult name.
Minulla on vaikeaa. = I am having a hard time. (impersonal, adjective in the partitive)

So for it is difficult for me to do X, you need the genitive:

Minun on vaikea tehdä se. = It’s difficult for me to do it.

Why is it nukahtaa and not nukkua?

Finnish makes a clear distinction:

nukkua = to sleep (the state)
nukahtaa = to fall asleep (the moment of going into sleep)

The sentence talks about the difficulty of falling asleep, not of sleeping in general, so:

Minun on vaikea nukahtaa yöllä.
= It is hard for me to fall asleep at night.

Saying minun on vaikea nukkua yöllä would mean:

It is hard for me to sleep at night (to remain asleep / get enough sleep), which is a slightly different idea.

Why is nukahtaa in the basic (infinitive) form and not something like nukahdan?

In the construction minun on vaikea + verb, that verb appears in the 1st infinitive (dictionary form):

on vaikea nukahtaa = is difficult to fall asleep
on helppo sanoa = is easy to say
on kiva nähdä = is nice to see

If you used nukahdan, you would be conjugating the verb (I fall asleep), which doesn’t fit grammatically after this adjective construction. The idea is:

minun on vaikea nukahtaa
≈ for me, to fall asleep is difficult

Why is yöllä in the inessive case, and does it mean “at night” in general?

Yöllä is the inessive form of yö (night):

yö (nominative) → yöllä (inessive: in / at night)

In time expressions, the inessive often corresponds to at / during:

päivällä = in/at the daytime
talvella = in (the) winter
yöllä = at night, during the night

In this sentence, yöllä means at night (in general), not one specific night. Context or extra words would specify a particular night, e.g.:

tänä yönä = tonight
viime yönä = last night

Can I put the clauses in the opposite order: Minun on vaikea nukahtaa yöllä, jos otan liian pitkän päiväunen?

Yes. Both orders are correct:

Jos otan liian pitkän päiväunen, minun on vaikea nukahtaa yöllä.
Minun on vaikea nukahtaa yöllä, jos otan liian pitkän päiväunen.

The meaning is the same. Starting with the jos-clause puts more focus on the condition; starting with minun on vaikea nukahtaa yöllä puts the focus first on the difficulty. Both are natural.