Learn Hardcore Finnish: Pelaaminen on minun lempiharrastukseni.

pelata (to play) → stem pelaa-
- -minen → pelaaminen “playing (games/sports)”
  Other examples: lukeminen (reading), kirjoittaminen (writing).

Why is the verb on used here? Could it be omitted or changed to olen?

You need the copula on (“is”). The subject pelaaminen is third person singular, so the correct form is on, not olen (“I am”). Finnish doesn’t normally drop the copula in standard language, so keep on.

Why is it minun and not minä?

Possessors appear in the genitive: minä → minun. So “my favorite hobby” is minun lempiharrastukseni.

Do I need both minun and the possessive ending -ni? Isn’t that redundant?

In standard Finnish you can:

use just the suffix: Pelaaminen on lempiharrastukseni (fully standard, somewhat concise/formal);
use both for clarity/emphasis: Pelaaminen on minun lempiharrastukseni (also standard). In colloquial speech people often drop the suffix and say: Pelaaminen on mun lempiharrastus. That last one is not recommended in formal writing.

What case is lempiharrastukseni in? It looks like it has an -n.

It’s nominative (predicate nominative). The sequence -kseni is not a case ending; it’s the noun’s stem + the possessive suffix -ni. The -n you see belongs to -ni. So the predicate agrees with the subject in nominative: Pelaaminen (nom.) on lempiharrastukseni (nom.).

Why does harrastus become harrastukseni? Where does the -kse- come from?

Nouns ending in -s (like harrastus) take a stem -kse- before many endings and suffixes:

harrastus → stem harrastukse-
- -ni → harrastukseni
  Compare: vastaus → vastaukseni, opetus → opetukseni.

Is lempiharrastus one word? Should there be a hyphen?

It’s a single compound word: lempi + harrastus → lempiharrastus. Finnish compounds are written together, and all endings/suffixes go at the very end: lempiharrastukseni.

Can I flip the word order to say the same thing?

Yes. These are all equivalent in meaning; word order mainly affects emphasis:

Pelaaminen on (minun) lempiharrastukseni. (Focus on “playing”)
(Minun) lempiharrastukseni on pelaaminen. (Focus on “my favorite hobby”)

Why do I sometimes see an adjective in the partitive, like hauskaa, after -minen words?

With adjectives as predicatives describing activities/abstracts, Finnish often uses the partitive:

Pelaaminen on hauskaa/vaikeaa. (“Playing is fun/difficult.”)
But when the predicative is a noun of classification (like “(my) favorite hobby”), it’s nominative: Pelaaminen on (minun) lempiharrastukseni.

Does pelata cover all the meanings of English “play”?

No. Finnish splits “play” three ways:

pelata = play games/sports: pelata jalkapalloa, pelata videopelejä
leikkiä = play (as children): lapset leikkivät
soittaa = play an instrument: soittaa kitaraa

Is there a difference between lempi- and suosikki-?

Both mean “favorite.”

lempi- is a bit literary/traditional: lempiruoka, lempiharrastus
suosikki- is very common and neutral: suosikkiruoka, suosikkiharrastus
Your sentence can be: Pelaaminen on suosikkiharrastukseni.

How do I say “one of my favorite hobbies”?

Use yksi + elative plural of the possessed noun:

Pelaaminen on yksi lempiharrastuksistani.
(= “Playing is one of my favorite hobbies.”)
Breakdown: lempiharrastus → lempiharrastuksista (elative plural) + -ni.

How do I say “his/her/their favorite hobby”?

“his/her”: (Hänen) lempiharrastuksensa
“their”: (Heidän) lempiharrastuksensa
The suffix -nsA is the same for both singular “his/her” and plural “their,” so the pronoun (hänen/heidän) is often added to avoid ambiguity.

Any pronunciation tips for this sentence?

Stress the first syllable of each word: PE-laa-mi-nen on MI-nun LEM-pi-har-RAS-tuk-se-ni.
Long sounds matter: aa in pelaaminen is long; rr is a long trill.
Pronounce every letter; Finnish spelling is phonemic.

What about pelaus or pelailu instead of pelaaminen?

pelaaminen is the neutral, general “playing (games/sports).”
pelaus appears in some set phrases/technical sports talk (e.g., hockey, “team’s defensive play”), but as a general activity noun, pelaaminen is safer.
pelailu suggests casual, leisurely playing (“messing around playing”), more colloquial.

Could I use the essive -na/-nä (e.g., harrastuksena) here?

Not in this copular classification. Use nominative for “X is (my) Y”:

Correct: Pelaaminen on (minun) lempiharrastukseni.
Essive -na/-nä means “as/in the role of”: Työskentelen opettajana (“I work as a teacher”). It’s not used for this “is my favorite hobby” pattern.