Learn Hardcore German: Beim Laufen im Park schwitze ich schnell, aber die frische Luft tut mir gut. - When running in the park I sweat quickly, but the fresh air does me good.

Beim is the standard contraction of bei dem. In normal spoken and written German, bei dem almost always becomes beim in this kind of phrase.

bei dem Laufen is grammatically possible but sounds overly heavy or contrastive here.
bei Laufen is wrong, because bei needs a dative noun after it, and Laufen here is being used as a noun (das Laufen), so it needs an article: bei dem Laufen → beim Laufen.

So the structure is:

bei (preposition, dative) + dem (dative article) + Laufen (noun)
→ contracted to beim Laufen = “while running / when I run” in this context.

Why is Laufen capitalized here instead of laufen?

In beim Laufen, Laufen is not a verb; it is a noun formed from a verb (a nominalized infinitive).

German often turns infinitives into nouns:

das Laufen – running
das Essen – eating / the food
das Lesen – reading

Whenever an infinitive is used as a noun, it:

Is capitalized.
Usually takes das as its article.
Can follow prepositions just like any other noun.

So beim Laufen literally means “at the running / during the running”, which idiomatically corresponds to “when (I am) running” / “while running” in English.

What case is beim Laufen, and why is that case used?

Beim Laufen is in the dative case.

The preposition bei always takes the dative.
Laufen is a noun here: das Laufen.
The dative of das Laufen with bei is bei dem Laufen → beim Laufen.

Pattern:

bei dem Essen → beim Essen
bei dem Arbeiten → beim Arbeiten
bei dem Laufen → beim Laufen

So the case is dative because the preposition bei requires it, not because of anything special about Laufen itself.

What case is im Park, and why is it im Park and not in den Park?

Im Park is also dative.

im is the contraction of in dem.
Park is masculine: der Park.
Dative singular masculine: dem Park.

We use in + dative when talking about location (where something is):

im Park = in dem Park → in the park (location)
in der Stadt – in the city
im Haus – in the house

We use in + accusative when talking about movement into something (where something is going):

in den Park gehen – to go into the park (movement)
in die Stadt fahren – to drive into the city

Here, you’re talking about running in the park as a location, not about going into the park, so in dem Park → im Park (dative) is correct.

Why is the word order schwitze ich and not ich schwitze after Beim Laufen im Park?

German main clauses follow the verb-second (V2) rule: the conjugated verb must be the second element in the sentence.

In this sentence:

First element (position 1): Beim Laufen im Park (a whole prepositional phrase)
Second element (position 2): schwitze (the conjugated verb)
Then the subject: ich
Then the adverb: schnell

So:

Beim Laufen im Park schwitze ich schnell … ✔
Beim Laufen im Park ich schwitze schnell … ✘ (verb is no longer in second position)

If you start with the subject instead, the verb comes second:

Ich schwitze beim Laufen im Park schnell … ✔

Both are correct; the choice affects emphasis and rhythm, not grammaticality.

Could I also say Ich schwitze beim Laufen im Park schnell, aber die frische Luft tut mir gut? Does it change the meaning?

Yes, that sentence is fully correct too.

Beim Laufen im Park schwitze ich schnell, …
→ Emphasis is slightly more on the situation “while running in the park”.
Ich schwitze beim Laufen im Park schnell, …
→ Emphasis starts with “I sweat quickly…”, with the situation added afterwards.

The meaning is essentially the same (you sweat quickly when running in the park, but the fresh air is good for you). The difference is only in information structure and emphasis, not in basic content.

What does tut mir gut mean exactly, and why is the verb tun used here?

Jemandem gut tun is a very common idiomatic expression meaning:

to be good for someone,
to do someone good,
to benefit someone.

In your sentence:

die frische Luft – the fresh air (subject)
tut – does (3rd person singular of tun)
mir – to me (dative)
gut – good

So die frische Luft tut mir gut literally means “the fresh air does me good”, and idiomatically “the fresh air is good for me.”

We use tun here because that’s simply the fixed pattern:

Das tut mir gut. – That does me good / That’s good for me.
Sport tut vielen Menschen gut. – Exercise is good for many people.

You would not say ist mir gut here; the natural expression is tut mir gut.

Why is it mir gut and not mich gut in die frische Luft tut mir gut?

Mir is the dative form of ich, and mich is the accusative form.

The verb phrase jemandem gut tun always takes the dative:

jemandem (dative) gut tun – to be good for someone

So:

mir (dative) – to me
dir (dative) – to you
ihm / ihr (dative) – to him / to her

Examples:

Das tut mir gut. – That’s good for me.
Frische Luft tut dir gut. – Fresh air is good for you.
Sport tut ihm gut. – Exercise is good for him.

Using mich (accusative) here would be grammatically wrong, because gut tun doesn’t take an accusative object; it takes a dative one.

Why is it die frische Luft and not just frische Luft?

Both are possible, but they are slightly different:

die frische Luft – the fresh air (more specific, a definite thing you have in mind)
frische Luft – (some) fresh air (more general or indefinite)

In context, die frische Luft often refers to the specific fresh air you are experiencing while outside / in the park, and German speakers commonly use the definite article here:

Die frische Luft tut mir gut. – This fresh air (that I’m breathing now) is good for me.

Without the article:

Frische Luft tut mir gut. – Fresh air (in general) is good for me.

Both are correct; the version in your sentence is just a bit more concrete.

Why is the adjective ending frische and not frischer or frischen in die frische Luft?

The form frische comes from regular adjective declension rules.

We have:

Noun: Luft (feminine)
Case: nominative (subject of the clause: the fresh air is doing something)
Article: die (definite article, nominative singular feminine)
Adjective: frisch

With a definite article in front of a feminine nominative singular noun, the adjective ending is -e:

die frische Luft – the fresh air
die schöne Stadt – the beautiful city
die neue Wohnung – the new apartment

If the case or article changed, the ending would change too:

mit der frischen Luft (dative)
wegen der frischen Luft (genitive)

But in your sentence, nominative + definite article + feminine noun → frische.

Does aber change the word order? Why is it aber die frische Luft tut mir gut and not something with the verb at the end?

Aber is a coordinating conjunction (like und, oder, denn), not a subordinating conjunction.

Coordinating conjunctions connect two main clauses and do not send the verb to the end.
Subordinating conjunctions (like weil, dass, wenn) introduce a subordinate clause and do send the conjugated verb to the end.

So:

… ich schwitze schnell, aber die frische Luft tut mir gut. ✔
(Both are main clauses; verb stays in second position in each.)

Compare:

… ich schwitze schnell, weil die frische Luft mir gut tut. ✔
(weil is subordinating, so tut moves to the end of its clause.)

Therefore, aber die frische Luft tut mir gut keeps normal main-clause word order: subject – verb – (objects / complements).