Learn Hardcore Finnish: Sunnuntaina haluan vain levätä kotona.

Because the sentence begins with the word, it’s capitalized: Sunnuntaina. In the middle of a sentence you’d normally see sunnuntaina with a lowercase s.

What case is the ending -na in Sunnuntaina, and what does it usually mean?

The ending -na/-nä is the essive case.

Typical uses:

Time: sunnuntaina = on Sunday, jouluna = at Christmas
Temporary state/role: opettajana = as a teacher, lapsena = as a child / when (I was) a child

In this sentence, sunnuntaina is a time adverbial: it answers “When?” → On Sunday.

Could I put sunnuntaina at the end and say Haluan vain levätä kotona sunnuntaina? Is there a difference?

Yes, that word order is grammatically correct:

Sunnuntaina haluan vain levätä kotona.
Haluan vain levätä kotona sunnuntaina.

Both can mean On Sunday I just want to rest at home.

The difference is in emphasis and flow:

Starting with Sunnuntaina slightly emphasizes when this happens.
Ending with sunnuntaina keeps the focus a bit more on the action resting at home, and adds on Sunday as extra info at the end.

Finnish word order is relatively flexible, but elements placed at the beginning of the sentence tend to be more in focus.

Why is there no minä? Shouldn’t it be Minä haluan vain levätä kotona?

In Finnish, subject pronouns (minä, sinä, hän, me, te, he) are usually dropped, because the verb ending already shows the person.

Haluan vain levätä kotona. = I just want to rest at home.
Minä haluan vain levätä kotona. = also correct, but minä adds emphasis: I (as opposed to someone else) want to rest.

In the given sentence, Sunnuntaina haluan vain levätä kotona, the -n ending in haluan already tells you the subject is I.

Why is it haluan and not something like halua?

Haluan is the 1st person singular form of the verb haluta (to want).

Conjugation of haluta (present tense):

minä haluan – I want
sinä haluat – you (sg.) want
hän haluaa – he/she wants
me haluamme – we want
te haluatte – you (pl.) want
he haluavat – they want

The personal ending -n marks the subject as I. The form halua without an ending would be used only in some special structures (e.g. en halua = I don’t want), but not as a main, affirmative verb meaning I want on its own.

Why is it haluan vain levätä and not something like haluan vain lepoa?

In Finnish, when you want to say “want to do something”, the typical pattern is:

haluta + FIRST INFINITIVE

So:

haluan levätä = I want to rest
haluan syödä = I want to eat
haluan lukea = I want to read

Lepo is a noun (rest as a thing), while levätä is the verb infinitive (to rest). Since haluan wants a verb for “want to do”, levätä is the correct form here, not lepoa.

What does vain mean here, and is it the same as vaan?

In this sentence, vain means “only / just”:

Haluan vain levätä. = I just want to rest / I only want to rest.

vain and vaan are different words:

vain = “only, just”
- Haluan vain levätä. – I only want to rest.
vaan = “but (rather) / but instead”
- En halua lähteä ulos, vaan haluan levätä kotona.
  = I don’t want to go out, but (instead) I want to rest at home.

Spelling matters: vain (only) vs vaan (but rather).

What form of the verb is levätä, and how is it conjugated?

Levätä is the basic infinitive (first infinitive) of the verb meaning to rest.

It’s a somewhat irregular type 4 verb (ending in -tä). Present tense conjugation:

minä lepään – I rest
sinä lepäät – you (sg.) rest
hän lepää – he/she rests
me lepäämme – we rest
te lepäätte – you (pl.) rest
he lepäävät – they rest

In haluan vain levätä, the verb stays in its infinitive form because it depends on haluan (I want to rest).

Why is it kotona and not koti or kotiin?

Finnish uses different forms of koti (“home”) to express location and direction:

koti – home (basic form)
kotona – at home (location, “where?”)
kotiin – (to) home (direction, “where to?”)
kotoa – (from) home (direction, “where from?”)

In the sentence, the meaning is “rest at home”, i.e. where you rest, so kotona is used.

Example contrasts:

Haluan levätä kotona. – I want to rest at home.
Haluan mennä kotiin. – I want to go (to) home.
Haluan lähteä kotoa. – I want to leave from home.

Is kotona also an essive like sunnuntaina, since it ends in -na?

No. Even though kotona also ends in -na, it’s not the same essive ending as in sunnuntaina.

koti has irregular local case forms:

kotona – at home (like an “inessive” meaning)
kotiin – to home (illative)
kotoa – from home (elative)

Historically there’s more going on, but for a learner it’s best to remember kotona / kotiin / kotoa as a special “home” set expressing place and direction, not as regular essive.

How would I say “On Sundays I just want to rest at home” (as a general habit)?

For a habitual / every Sunday meaning, Finnish usually uses sunnuntaisin:

Sunnuntaisin haluan vain levätä kotona.
= On Sundays I just want to rest at home.

So:

sunnuntaina = on (one particular) Sunday
sunnuntaisin = on Sundays (in general / usually)

How would this sentence look in the negative, like “On Sunday I don’t want to rest at home”?

To negate haluan, you use the negative verb en and the verb stem without the personal ending:

Sunnuntaina en halua levätä kotona.
= On Sunday I don’t want to rest at home.

Pattern:

haluan → en halua
haluat → et halua
haluaa → ei halua, etc.

Notice that levätä stays in the infinitive; only haluta is negated.