Learn Hardcore Finnish: Ennen treeniä teen rauhallista venyttelyä, jotta lihas pysyy rentona. - Before the workout I do gentle stretching so that the muscle stays relaxed.

Using nominative (ennen treeni) would be ungrammatical, and ennen treenin sounds wrong in modern standard Finnish; you only get a genitive with something like ennen treenin alkua (before the start of the workout).

So the pattern to remember is:

ennen + partitive → ennen treeniä, ennen lomaa, ennen aamiaista

But in English it’s before the workout, which sounds definite. Why is the Finnish treeniä (partitive) and not some “definite” form?

Finnish doesn’t have articles (a / the) or a dedicated “definite” case. Definiteness is expressed through context, word order, or other means.

Here, treeniä is in the partitive only because ennen requires the partitive, not because the workout is indefinite.

So:

ennen treeniä can mean before a workout or before the workout, depending on context.
The partitive here is purely driven by the preposition ennen, not by definiteness.

Why is it teen rauhallista venyttelyä instead of something like teen rauhallisen venyttelyn?

In teen rauhallista venyttelyä, the object venyttelyä is in the partitive, which gives the idea of:

an ongoing / unbounded activity
some amount of stretching, not a single, clearly finished event

If you said teen rauhallisen venyttelyn, that sounds like:

a single, clearly delimited stretching session, often one specific “stretching routine” you complete.

So:

Teen rauhallista venyttelyä ≈ I do some calm stretching / I (habitually) do calm stretching.
Teen rauhallisen venyttelyn ≈ I do/perform a specific calm stretching session (and finish it).

For general habits and activities, the partitive object (venyttelyä) is the most natural.

Why is rauhallista (calm) in the partitive form here?

Rauhallista is an adjective describing venyttelyä.

In Finnish, adjectives agree in case and number with the nouns they modify. Since:

venyttelyä is partitive singular,
the adjective must also be partitive singular: rauhallista.

So you get:

rauhallinen venyttely (nominative)
rauhallista venyttelyä (partitive)

The case of the adjective is driven by the case of the noun, not by any special rule about the adjective itself.

Why is venyttelyä in the partitive case?

Venyttelyä is the object of the verb teen (I do).

In Finnish, objects are often in the partitive when:

The action is ongoing, habitual, or not seen as completed, or
You’re talking about an indefinite amount or some of something.

Here, teen rauhallista venyttelyä describes an ongoing / habitual type of activity, not a single, completed stretching event. That is exactly the situation where the partitive object is used.

Compare:

Join veden. – I drank the (whole) water. (total object, accusative/genitive)
Join vettä. – I drank (some) water. (partitive object, unbounded)

Likewise:

Teen venyttelyn. – I do (and complete) one specific stretching session.
Teen venyttelyä. – I do some stretching (activity).

Could I say Venyttelen rauhallisesti ennen treeniä instead? What’s the difference?

Yes, Venyttelen rauhallisesti ennen treeniä is perfectly natural and means essentially the same thing:

Teen rauhallista venyttelyä – literally I do calm stretching (as a noun phrase).
Venyttelen rauhallisesti – I stretch calmly (using the verb venytellä).

Using venytellä makes the sentence a bit more verb‑y and less nominal. The nuance:

Teen rauhallista venyttelyä can sound a touch more descriptive or exercise‑routine‑like.
Venyttelen rauhallisesti is a straightforward description of the action.

Both are correct and idiomatic.

What exactly does jotta mean, and how is it different from että or koska?

Jotta introduces a clause of purpose or desired result. It’s close to English so that / in order that.

In the sentence:

jotta lihas pysyy rentona = so that the muscle stays relaxed

The main contrasts:

jotta → purpose / intended outcome
- Teen venyttelyä, jotta lihas pysyy rentona.
  I do stretching so that the muscle stays relaxed.
että → more neutral “that”, often reporting content
- Sanoin, että lihas pysyy rentona.
  I said that the muscle stays relaxed.
koska → cause, like “because”
- Teen venyttelyä, koska lihas on kireä.
  I stretch because the muscle is tight.

So jotta is the right choice when you want to express purpose/goal.

Why is there a comma before jotta?

Finnish normally separates independent clauses and many subordinate clauses with a comma.

Here you have:

Ennen treeniä teen rauhallista venyttelyä,
jotta lihas pysyy rentona.

The jotta‑clause is a subordinate clause of purpose attached to the first clause, so it is separated by a comma:

[Main clause] , jotta [subordinate purpose clause].

This is a standard punctuation rule in Finnish.

Why is it lihas (singular) and not lihakset (plural), when I presumably have many muscles?

Finnish often uses a singular noun to talk about something in a general or generic sense.

Here, lihas can be understood as:

the muscle in a generic sense, or
“each muscle” / “the muscle in question”

So:

jotta lihas pysyy rentona ≈ so that the muscle stays relaxed
(understood as “so that your muscle(s) stay relaxed”).

You can say:

jotta lihakset pysyvät rentoina – so that the muscles stay relaxed,

but the singular lihas is very natural when speaking generally about what happens to muscle tissue during exercise or stretching.

Why don’t we say minun lihas or lihakseni to mark “my muscle”?

In Finnish, it’s very common to omit possessive marking for things that are clearly part of your own body or otherwise obviously yours from context.

So instead of:

venyttelen minun lihastani
venyttelen lihastani,
venyttelen lihaksiani,

you typically just say:

venyttelen, jotta lihas pysyy rentona.

Listeners automatically understand that the muscle belongs to the speaker (or the subject) because of the context. Adding minun or a possessive suffix (-ni) here usually sounds overly explicit or unnatural, unless you are emphasizing whose muscle we are talking about.

Why is the verb pysyy and not pysyä or something else?

Pysyä is the infinitive (“dictionary form”) to stay / to remain.

In the sentence, the subject is lihas (muscle), which is 3rd person singular, so the verb is conjugated to match:

pysyä → pysyy (3rd person singular, present tense)

Basic pattern for pysyä in the present:

(minä) pysyn – I stay
(sinä) pysyt – you stay
(hän / se / lihas) pysyy – he/she/it/muscle stays
(me) pysymme – we stay
(te) pysytte – you (pl) stay
(he / ne / lihakset) pysyvät – they / muscles stay

What form is rentona, and why is that used instead of something like rento?

Rentona is the essive case of rento (relaxed).

rento → basic form (nominative)
rentona → essive singular

The essive case (-na / -nä) is used to describe:

a state or role: as X, in the state of X

Here:

lihas pysyy rentona = the muscle stays in a relaxed state / stays relaxed.

If you said lihas on rento, that’s fine, but it’s more like a static description: the muscle is relaxed.
Pysyy rentona emphasizes remaining in that state.

Is rentona an adverb here, like “relaxedly”?

Grammatically, rentona is not an adverb; it’s a noun/adjective in the essive case.

Functionally, it behaves a bit like a predicative complement: it tells you in what state the subject is remaining:

lihas pysyy rentona
= the muscle (subject)
- stays (verb)
  - in a relaxed state (essive complement)

If you wanted a true adverb (modifying how something is done), you’d use something like rauhallisesti (calmly). But rentona here is describing the state of the muscle, not the manner of an action.

Can I change the word order, for example to Teen rauhallista venyttelyä ennen treeniä, jotta lihas pysyy rentona?

Yes. Finnish word order is quite flexible, and your version is natural:

Teen rauhallista venyttelyä ennen treeniä, jotta lihas pysyy rentona.

The difference is mainly in emphasis:

Ennen treeniä teen rauhallista venyttelyä…
– Slight emphasis on before the workout.
Teen rauhallista venyttelyä ennen treeniä…
– Slight emphasis on the fact that I do calm stretching, and when I do it is added afterwards.

Both are correct and sound native-like.

Could I say jotta lihakset pysyvät rentoina instead of jotta lihas pysyy rentona?

Yes, and it would also be natural:

jotta lihas pysyy rentona – so that the muscle stays relaxed (generic singular)
jotta lihakset pysyvät rentoina – so that the muscles stay relaxed (explicit plural)

Note that rentoina is then also in the plural essive to agree with lihakset:

lihas – rentona
lihakset – rentoina

Both variants are correct; the original just uses a generic singular, which is very typical in Finnish.