Learn Hardcore German: Seitdem ich jeden Morgen im Park trainiere, fühle ich mich besser. - Since I train in the park every morning, I feel better.

In a main clause, the finite verb is in second position:
- Ich trainiere jeden Morgen im Park.
In a subordinate clause introduced by a conjunction (like weil, dass, obwohl, seitdem), the finite verb moves to the end of the clause:
- Seitdem ich jeden Morgen im Park trainiere, …

So the pattern here is:

Seitdem (conjunction)
ich (subject)
jeden Morgen im Park (other elements)
trainiere (finite verb at the end)

Why does the second part say fühle ich mich besser instead of ich fühle mich besser?

The second part is a main clause, but it comes after a subordinate clause that stands at the beginning of the sentence.

In German main clauses, the finite verb must be in second position. The whole subordinate clause Seitdem ich jeden Morgen im Park trainiere counts as position 1 in the overall sentence structure.

So the word order is:

Seitdem ich jeden Morgen im Park trainiere, (entire subordinate clause = position 1)
fühle (finite verb = position 2)
ich (subject)
mich besser (rest of the sentence)

That’s why we get fühle ich mich besser, not ich fühle mich besser, in this context.
If you start directly with the main clause, you would indeed say:

Ich fühle mich besser.

What is the difference between seitdem and seit here? Could I say Seit ich jeden Morgen im Park trainiere, fühle ich mich besser?

Both seitdem and seit can introduce a time clause meaning since (in the sense of “since the time when …”).

Seitdem is explicitly a subordinating conjunction here.
Seit can be a preposition (seit gestern = since yesterday) or a conjunction (Seit ich… = since I…).

In your sentence, you can say:

Seit ich jeden Morgen im Park trainiere, fühle ich mich besser.

This is correct and very natural. Nuance:

Seitdem as a conjunction feels slightly more explicit about “since that time onwards”.
Seit is a bit shorter and arguably a bit more common in everyday speech.

Meaning-wise, in this sentence, they’re basically interchangeable.

Why is the present tense (trainiere) used instead of something like ich habe im Park trainiert or an English-style continuous form (“I have been training”)?

German often uses the present tense (Präsens) to express actions that:

started in the past, and
are still continuing in the present.

English would typically use a present perfect continuous (“I have been training”), but German does not have a dedicated continuous tense. So:

Seitdem ich jeden Morgen im Park trainiere, fühle ich mich besser.
= Since I have been training in the park every morning, I feel better.

Using Perfekt (present perfect) here would not be natural:

✗ Seitdem ich jeden Morgen im Park trainiert habe, fühle ich mich besser.
This suggests something more completed and sounds off in this context.

So trainiere (present tense) is the natural way to express an ongoing, repeated action that started in the past and continues.

Why is it jeden Morgen and not jeder Morgen?

Jeden Morgen is the masculine accusative singular form of jeder Morgen.

For time expressions like every day, every morning, every evening, German very often uses the accusative case:

jeden Tag (every day)
jedes Jahr (every year)
jede Woche (every week)
jeden Morgen (every morning)

Declension of jeder in the singular:

Masculine: jeder (nom.), jeden (acc.)
Feminine: jede (nom./acc.)
Neuter: jedes (nom./acc.)

Since Morgen is masculine and is used here as a time adverbial in the typical accusative pattern, we get jeden Morgen.

What case is im Park, and why is it im instead of in dem?

Im Park is dative case, because of the preposition in used with a location (where?).

in
- dative = location (in dem Park = in the park – “where?”)
in
- accusative = direction (in den Park = into the park – “where to?”)

Here we are expressing where the training takes place (location), so it’s dative:

in dem Park → contracted to im Park

Im is simply the standard contraction of:

in
- dem → im

Is the word order jeden Morgen im Park fixed? Could I say im Park jeden Morgen instead?

German has a preferred order of adverbials often taught as Time–Manner–Place (TMP):

Zeit – Art und Weise – Ort
(time – manner – place)

In your sentence:

jeden Morgen = time
im Park = place

So jeden Morgen im Park follows this rule nicely: time → place.

You can say im Park jeden Morgen, and it is grammatically correct, but it slightly shifts the emphasis:

Seitdem ich im Park jeden Morgen trainiere, …
sounds a bit like you’re highlighting in the park more strongly.

Most neutral and typical would be:

Seitdem ich jeden Morgen im Park trainiere, …

Why is fühlen used with mich (reflexive) here? Why not just ich fühle besser?

In German, the verb sich fühlen (reflexive) is used to talk about how someone feels (emotionally or physically):

Ich fühle mich besser. = I feel better.
Wie fühlst du dich heute? = How do you feel today?

Using fühlen without the reflexive pronoun usually means to touch, to feel something with your hands:

Ich fühle die kalte Wand. = I feel the cold wall (with my hand).

So:

✗ Ich fühle besser. – sounds wrong in German in this sense.
✓ Ich fühle mich besser. – correct for “I feel better.”

That’s why the sentence says fühle ich mich besser.

Why is the reflexive pronoun mich placed after ich and the verb: fühle ich mich besser? Could it go elsewhere?

In a simple main clause, the usual order is:

Subject – finite verb – reflexive pronoun – other elements

Example:

Ich fühle mich besser.

In your sentence, the main clause word order is affected by the initial subordinate clause, but the internal order is still the same:

fühle (verb – must be 2nd position in the main clause)
ich (subject)
mich (reflexive pronoun)
besser (adverb)

So:

Seitdem ich … trainiere, | fühle ich mich besser.

You can’t move mich before ich in standard German:

✗ fühle mich ich besser – wrong.
✓ fühle ich mich besser – correct.

In general, unstressed pronouns (like mich, dich, ihn) tend to come early in the middle field, shortly after the verb and subject.

Could I use seitdem differently, like Seitdem trainiere ich jeden Morgen im Park, fühle ich mich besser?

You’re mixing two different uses of seitdem:

Conjunction use (as in your original sentence):
- Seitdem ich jeden Morgen im Park trainiere, fühle ich mich besser.
  Here seitdem introduces a subordinate clause with the verb at the end.
Adverbial use (like “since then”):
- Ich trainiere jeden Morgen im Park, seitdem fühle ich mich besser.
  Here seitdem is an adverb meaning “since then”, and the verb stays in second position:
- …, seitdem fühle ich mich besser.

Your suggested “Seitdem trainiere ich jeden Morgen im Park, …” is odd if you then add another clause, because seitdem in that position is understood as “since then”, not “since the time when…”. It would usually stand alone as a main clause:

Seitdem trainiere ich jeden Morgen im Park.
= Since then I’ve been training in the park every morning.

To keep the original meaning (“since I started training every morning in the park”), use either:

Seitdem ich jeden Morgen im Park trainiere, fühle ich mich besser.
or
Seit ich jeden Morgen im Park trainiere, fühle ich mich besser.

Can I leave out the ich in the subordinate clause and say Seitdem jeden Morgen im Park trainiere, fühle ich mich besser?

No. In German, you generally must state the subject explicitly in each clause.

Seitdem ich jeden Morgen im Park trainiere, … – correct.
✗ Seitdem jeden Morgen im Park trainiere, … – incorrect; the subject ich is missing.

German does not allow dropping the subject the way some languages do (like Spanish or Italian). Each finite verb needs an explicit subject, unless it’s an impersonal construction (like es regnet).

Why is there a comma before fühle ich mich besser?

Because German always separates a subordinate clause from the main clause with a comma.

Structure:

Seitdem ich jeden Morgen im Park trainiere, (subordinate clause introduced by seitdem)
fühle ich mich besser. (main clause)

Rule:

Subordinate clause before the main clause → comma at the end of the subordinate clause.
Subordinate clause inside or after the main clause → commas around / before it.

So the comma is mandatory here under standard punctuation rules.