Learn Hardcore Dutch: Door elke dag te rekken, worden je spieren soepel. - By stretching every day, your muscles become flexible.

door ... te + infinitive = “by …-ing,” expressing means or cause: the mechanism that leads to the result. Example: Door elke dag te rekken, worden je spieren soepel = “By stretching every day, your muscles become supple.”
om ... te + infinitive = “in order to,” expressing purpose or intention. Example: Ik rek elke dag om soepeler te worden = “I stretch every day in order to become more supple.”
They are not interchangeable.

Is the comma after the introductory door … te clause required?

No. It’s optional. Dutch often places a comma after a long or fronted adverbial for readability:

With comma: Door elke dag te rekken, worden je spieren soepel.
Without comma: Door elke dag te rekken worden je spieren soepel.

Why does the finite verb worden come right after the comma?

Main-clause verb-second (V2) word order. The fronted door … te phrase occupies the first position, so the finite verb (worden) must be second, followed by the subject (je spieren).
Alternate order without fronting: Je spieren worden soepel door elke dag te rekken.

Why is it worden and not wordt?

Agreement. Je spieren is plural, so the verb is plural: worden.
Singular example: Door elke dag te rekken, wordt je spier soepel.

Is worden here a passive?

No. This is not the passive; it’s worden as a linking verb meaning “become,” followed by an adjective (soepel).
Passive would be worden + past participle, e.g., De spieren worden gerekt = “The muscles are being stretched.”

Could I use zijn instead of worden?

You can, but it changes the meaning.

worden = a change of state: “become.”
zijn = a state: “are.”
So: Door elke dag te rekken, zijn je spieren soepel suggests the resulting state (they are supple); … worden … emphasizes the process of becoming supple.

Where should adverbs like elke dag go in a door … te clause? Is Door te rekken elke dag okay?

Place adverbs and objects before te + infinitive: Door elke dag te rekken is natural.
Door te rekken elke dag is awkward and usually considered wrong in standard Dutch.
Other examples: Door regelmatig te oefenen, Door niet elke dag te rekken.

Can I move the door … te phrase to the end?

Yes: Je spieren worden soepel door elke dag te rekken.
Fronting it (as in the original) simply emphasizes the cause/means.

Do I need a reflexive with rekken? What about strekken or the loanword stretchen?

In sports contexts, intransitive rekken (“to do stretching”) is fine: Ik rek elke dag.
You will also see reflexive forms: je (uit)rekken: Ik rek me (uit) = “I stretch (myself).”
Strekken often means “to stretch (out) a body part”: Strek je armen. In the set phrase rekken en strekken, both appear.
The loanword stretchen is common in gyms: Door elke dag te stretchen… (infinitive: stretchen; past: stretchte; participle: gestretcht).

What’s the nuance of je versus jouw and uw in je spieren?

je = unstressed “your,” neutral and very common: je spieren.
jouw = stressed/emphatic “your”: jouw spieren (contrastive: your muscles, not someone else’s).
uw = formal “your” (to strangers or in formal writing): uw spieren.

Is the sentence ambiguous about who is doing the stretching?

Slightly. The main-clause subject is je spieren, but logically the person (you) does the stretching. Dutch allows this with door … te clauses, but some style guides prefer avoiding potential mismatches. Clearer options:

Als je elke dag rekt, worden je spieren soepel.
Door zelf elke dag te rekken, maak je je spieren soepel.

Should it be soepel or soepeler?

Both are possible, but they say different things:

… worden soepel = they become supple (reach a supple state).
… worden soepeler = they become more supple (comparative).
In many real-life contexts, soepeler is the more natural claim about progress.

What’s the difference between soepel, lenig, and flexibel?

soepel: supple, smooth, limber (common for muscles/movement).
lenig: limber/pliable, often used for a person’s body/abilities (you can be lenig).
flexibel: flexible; widely used metaphorically (schedules, rules), but also physically.

Can I use omdat (“because”) instead of door here?

Yes, with a full clause: Omdat je elke dag rekt, worden je spieren soepel.
Nuance: door highlights the means/mechanism (“by …-ing”), while omdat gives a reason (“because”).

How do I negate the door … te clause?

Place niet/geen before te + infinitive:

Door niet elke dag te rekken, worden je spieren stijf. (“By not stretching every day, your muscles become stiff.”)
Door geen pauzes te nemen, … (if you need “no” rather than “not”).

Any quick pronunciation tips for key words?

Approximate English-like hints:

Door: “dohr”
elke: “EL-kuh”
rekken: “REK-kən” (clear double k)
worden: “WOR-dən”
je: unstressed “yuh”
spieren: “SPEE-rən”
soepel: “SOO-pəl” (oe = “oo” in “food”)