Learn Hardcore German: Nach jeder Trainingseinheit fühle ich mich müde, aber zufrieden. - After each training session I feel tired but content.

Trainingseinheit is feminine: die Trainingseinheit.
Feminine jede- in the singular declines like this:
- Nominative: jede Trainingseinheit (every training session – as subject)
- Accusative: jede Trainingseinheit
- Dative: jeder Trainingseinheit
- Genitive: jeder Trainingseinheit

After nach you must use dative, so it has to be jeder Trainingseinheit, not jede.

What gender and plural does Trainingseinheit have, and what does it mean exactly?

Gender: die Trainingseinheit (feminine)
Plural: die Trainingseinheiten

Meaning-wise, Trainingseinheit is a single training session / workout session, i.e. one complete unit of training, typically with a clear beginning and end.

It’s more specific than das Training, which can be uncountable and mean training in general or a training period.

Could I also say nach dem Training or nach jedem Training instead of nach jeder Trainingseinheit?

Yes, both are possible and natural, with small nuances:

Nach dem Training fühle ich mich müde, aber zufrieden.
→ After the training (this particular training), I feel tired but content.
Nach jedem Training fühle ich mich müde, aber zufrieden.
→ After every training session, I feel tired but content.
Here Training is neuter: das Training → nach jedem Training (dative).
Nach jeder Trainingseinheit…
Emphasises each individual, discrete session as a “unit” of training. It sounds a bit more technical/sports-specific.

All three are correct; choice depends on style and what you want to highlight.

Why is the word order Nach jeder Trainingseinheit fühle ich mich… and not Nach jeder Trainingseinheit ich fühle mich…?

In German main clauses, the conjugated verb must be in second position (the famous V2 rule).

Nach jeder Trainingseinheit = first sentence element (the whole time phrase counts as one unit)
So the conjugated verb fühle must come next (second position)
Then comes the subject ich

So:

✅ Nach jeder Trainingseinheit fühle ich mich müde, aber zufrieden.
✅ Ich fühle mich nach jeder Trainingseinheit müde, aber zufrieden.
❌ Nach jeder Trainingseinheit ich fühle mich … (verb is not in second position)

Why do we say ich fühle mich and not just ich fühle here?

German makes a distinction:

fühlen (non‑reflexive): to feel something with your senses
- Ich fühle den Wind. – I feel the wind.
- Kannst du das fühlen? – Can you feel that?
sich fühlen (reflexive): to feel in a certain state (physically or emotionally)
- Ich fühle mich müde. – I feel tired.
- Er fühlt sich krank. – He feels ill.
- Wir fühlen uns gut. – We feel good.

In your sentence, you’re talking about your state, so German needs the reflexive form sich fühlen → ich fühle mich.
Ich fühle müde on its own is ungrammatical.

Why is it mich and not mir in ich fühle mich?

Reflexive pronouns change with case. For ich you have:

Accusative: mich
Dative: mir

In ich fühle mich müde:

mich is the direct object (accusative) of fühle
There is no other accusative object in the sentence
Therefore the reflexive pronoun appears in the accusative: mich

You would get mir (dative) when there is another, “more important” accusative object:

Ich koche mir einen Kaffee.
- mir = dative (reflexive: for myself)
- einen Kaffee = accusative object

With sich fühlen, it’s almost always used with accusative: ich fühle mich, du fühlst dich, er fühlt sich, etc.

What is the difference between ich fühle mich müde and ich bin müde?

Both are correct and often interchangeable, but the nuance is:

Ich bin müde.
States a fact about your condition: I am tired.
Ich fühle mich müde.
Emphasises your subjective experience of the state: I feel tired.
It can sound a bit more perceptual / introspective, like you are focusing on how you’re experiencing your body or mood.

In everyday speech, Ich bin müde is more common and simpler.
Ich fühle mich müde works well in contexts where you’re explicitly talking about how you feel in your body/yourself, often together with other adjectives:

Ich fühle mich müde, aber zufrieden.
Ich fühle mich heute etwas schlapp.

Why is there a comma before aber?

In German, when aber connects two (more or less) independent clauses, you must put a comma before it.

Here the full underlying structure is:

Nach jeder Trainingseinheit fühle ich mich müde, aber (ich fühle mich) zufrieden.

The second clause omits the repeated ich fühle mich, but grammatically it is still treated like a separate clause joined by aber, so the comma stays:

✅ … fühle ich mich müde, aber zufrieden.

This is a standard rule: a comma before aber when it links clauses.

Could I say müde, aber ich bin zufrieden instead of müde, aber zufrieden?

Yes, this is possible:

Nach jeder Trainingseinheit fühle ich mich müde, aber ich bin zufrieden.

Differences:

müde, aber zufrieden
- Short and compact.
- Presents two adjectives side by side describing your state.
- The verb and subject ich fühle mich are understood for the second adjective and left out.
müde, aber ich bin zufrieden
- Slightly more explicit, with a visible contrast: I feel tired, but I am satisfied.
- Stylistically a bit heavier, but perfectly correct.

You would normally prefer the shorter original version unless you want to strongly emphasise the contrast with ich bin.

What nuance does zufrieden have compared to English happy?

zufrieden usually means:

content, satisfied, at peace with a result or situation

It does not necessarily mean:

excited, overjoyed, or very cheerful

Examples:

Ich bin mit dem Ergebnis zufrieden.
I’m satisfied with the result. (not: thrilled, just content)

Compared to English:

happy is often stronger and more emotional.
zufrieden is more like content / satisfied, sometimes with a calm, balanced feeling.

In your sentence, müde, aber zufrieden suggests: physically tired, but emotionally content with what you achieved in training.

Can the time phrase nach jeder Trainingseinheit go somewhere else in the sentence?

Yes. These are all correct:

Nach jeder Trainingseinheit fühle ich mich müde, aber zufrieden.
– Very natural; fronting the time element is common in German.
Ich fühle mich nach jeder Trainingseinheit müde, aber zufrieden.
– Also very natural; subject first, time phrase in the middle.
Ich fühle mich müde nach jeder Trainingseinheit, aber zufrieden.
– Grammatically okay, but less idiomatic; the time phrase sounds a bit tacked on.

Typical preferences in German:

Time phrases often go either at the beginning or just before the verb cluster.
So (1) and (2) are the best choices stylistically.

Is Trainingseinheit a common, natural word in everyday German, or would people say something else?

Trainingseinheit is quite common and natural, especially in sports, fitness, and coaching contexts.

Examples:

Wir haben heute drei Trainingseinheiten. – We have three training sessions today.
Die nächste Trainingseinheit ist am Mittwoch. – The next training session is on Wednesday.

In casual speech, people also often say:

Nach dem Training… – After (the) training…
Nach jeder Trainingsstunde… – After every training class/lesson…
Nach jedem Workout… – After every workout… (using the English loanword)

So Trainingseinheit is perfectly fine and idiomatic, just slightly more structured/technical, like talking about specific units on a training plan.