Learn Hardcore Finnish: Veljeni rentoutuu iltaisin pelaamalla videopeliä. - My brother relaxes in the evenings by playing a video game.

Veli means brother, and the ending -ni is a possessive suffix meaning my.
So veljeni literally is brother‑my, i.e. my brother.

Finnish often attaches the possessor directly to the noun instead of using a separate word like English my or German mein.

You can also say:

minun veljeni = my brother (more explicit/emphatic)

But just veljeni on its own is perfectly normal and often more natural in standard Finnish.

Can veljeni also mean my brothers? How can I tell if it’s singular or plural?

Yes, veljeni can mean either:

my brother (singular), or
my brothers (plural).

They look the same in the nominative with the -ni ending. You tell the number from context, especially from the verb:

Veljeni on sairas. = My brother is sick. (singular verb on)
Veljeni ovat sairaita. = My brothers are sick. (plural verb ovat)

In your sentence, Veljeni rentoutuu…, the verb rentoutuu is singular, so it clearly means my brother (one person).

What form of the verb is rentoutuu, and how would I say I relax, we relax, etc.?

The basic verb is rentoutua = to relax (to become relaxed).

Rentoutuu is:

3rd person singular
present tense
indicative mood

So rentoutuu = he/she relaxes or (someone) relaxes.

Present tense forms:

minä rentoudun = I relax
sinä rentoudut = you relax (singular)
hän rentoutuu = he/she relaxes
me rentoudumme = we relax
te rentoudutte = you relax (plural)
he rentoutuvat = they relax

The stem changes a bit (rentoutu- → rentoudu-) because of consonant gradation, which is a regular Finnish sound pattern.

Is rentoutuu like a reflexive verb (relaxes himself), and is there another verb for to relax something?

Rentoutua is intransitive: it means that the subject itself becomes relaxed. Finnish doesn’t need a separate reflexive pronoun like himself here.

Veljeni rentoutuu. = My brother relaxes / becomes relaxed.

If you want to say to relax something/someone, you use the related transitive verb rentouttaa:

Musiikki rentouttaa veljeni. = Music relaxes my brother.
Hieronta rentouttaa lihakset. = Massage relaxes the muscles.

So:

rentoutua = to become relaxed (subject changes state)
rentouttaa = to relax something (object changes state)

Why is it iltaisin and not illalla? What is the difference?

Both relate to ilta (evening), but they express different ideas:

illalla = in the evening / on a (particular) evening
- Focus on one specific time:
  - Tulen illalla. = I’ll come in the evening (today/that day).
iltaisin = in the evenings / in the evenings in general
- Describes a regular, repeated habit:
  - Veljeni rentoutuu iltaisin. = My brother relaxes in the evenings (as a routine).

The ending -sin is used to form adverbs that mean habitually at that time:

aamuisin = in the mornings
öisin = at night(s)
viikonloppuisin = on weekends (habitually)

What exactly is pelaamalla? Is it like the English by playing?

Yes, pelaamalla corresponds very closely to by playing.

Grammatically, pelaamalla is:

the 3rd infinitive of pelata (to play)
in the adessive case (ending -lla / -llä)

Pattern:

verb pelata → 3rd infinitive stem pelaama- → add adessive: pelaamalla

This -malla/-mällä form expresses means or manner: doing something by doing something else.

So:

rentoutua pelaamalla videopeliä = to relax by playing a video game

More examples with the same structure:

Rentoudun lukemalla. = I relax by reading.
Opin parhaiten kirjoittamalla. = I learn best by writing.
Hän laihtui juoksemalla. = He/She lost weight by running.

Could I also say Veljeni rentoutuu iltaisin, kun hän pelaa videopeliä? Does it mean the same thing?

Yes, that sentence is correct and natural:

Veljeni rentoutuu iltaisin, kun hän pelaa videopeliä.
= My brother relaxes in the evenings when he plays a video game.

The difference is subtle:

pelaamalla videopeliä emphasizes means/method: he relaxes *by playing*.
kun hän pelaa videopeliä emphasizes time/condition: he relaxes *when he plays*.

In many contexts, they’ll feel almost the same, but pelaamalla is more compact and strongly highlights the activity as the way he relaxes.

Why is videopeliä in the partitive case, and not videopeli or videopelin?

Videopeliä is the partitive singular of videopeli (video game).

Finnish very often uses the partitive for the object when the action is:

ongoing, incomplete, or open‑ended
a general activity rather than finishing one whole, countable thing

The verb pelata (to play (a game, sport)) normally takes a partitive object:

pelata jalkapalloa = to play football
pelata shakkia = to play chess
pelata korttia = to play cards
pelata videopeliä = to play a video game

So videopeliä here presents playing as an ongoing hobby/activity, not completing some specific single game from start to finish.

If you used videopelin, it would suggest playing one entire game (e.g. We played the game through once), and even then you’d usually add something like:

Pelasimme videopelin läpi. = We played the video game through (to the end).

Is videopeliä singular or plural? How would I say video games in general?

Videopeliä is singular partitive: a video game in the sense of some video game / the activity of playing a video game.

For video games in general (plural), you would use the partitive plural:

videopelejä

Examples:

Veljeni rentoutuu iltaisin pelaamalla videopeliä.
= My brother relaxes in the evenings by playing a video game (not specifying which, just the hobby).
Veljeni rentoutuu iltaisin pelaamalla videopelejä.
= My brother relaxes in the evenings by playing video games (various games in general).

Can I change the word order, for example Iltaisin veljeni rentoutuu pelaamalla videopeliä? Does that change the meaning?

Yes, Finnish word order is quite flexible, and this sentence can be rearranged:

Veljeni rentoutuu iltaisin pelaamalla videopeliä.
Iltaisin veljeni rentoutuu pelaamalla videopeliä.

Both are correct and mean essentially the same thing.

The difference is mainly in emphasis:

Starting with Veljeni highlights my brother.
Starting with Iltaisin highlights the time: In the evenings, my brother relaxes…

The basic meaning (who does what, when, how) stays the same.

Why don’t we have a separate he (hän) as the subject? Why isn’t it Hän rentoutuu iltaisin pelaamalla videopeliä?

In the given sentence, the subject is already veljeni (my brother), so adding hän would be redundant.

Finnish works differently from English:

The verb ending already shows the person and number.
A subject noun or phrase (veljeni) is usually enough.
Personal pronouns (hän, he, etc.) are used mainly for emphasis, contrast, or when there is no other subject noun.

By contrast:

Hän rentoutuu iltaisin pelaamalla videopeliä.
= He/She relaxes in the evenings by playing a video game.
Here hän is the subject.

In your sentence, Veljeni rentoutuu…, the subject is clear without hän, so Finnish simply doesn’t add it.