Learn Hardcore German: Bergsteigen braucht viel Ausdauer, deshalb trainiere ich dreimal pro Woche. - Mountaineering needs a lot of stamina, therefore I train three times a week.

Bergsteigen is originally a verb (bergsteigen = to climb mountains), but here it is used as a noun-like word (a nominalized infinitive).

In German, when an infinitive verb is used as a general activity or concept, it is written:

with a capital letter
without an article (often)
Examples:
Schwimmen macht Spaß. – Swimming is fun.
Lesen bildet. – Reading educates.
Bergsteigen braucht viel Ausdauer. – Mountain climbing requires a lot of stamina.

So in this sentence Bergsteigen functions grammatically as a singular neuter noun (subject of the sentence), which is why the verb is braucht (3rd person singular).

Why is the verb braucht (3rd person singular) and not brauchen?

The verb has to agree with the grammatical subject.

Subject: Bergsteigen (a singular activity / concept)
Verb: brauchen (to need / require)
3rd person singular form: braucht

So:

Bergsteigen braucht viel Ausdauer. – Mountain climbing requires a lot of stamina.

If the subject were plural, you would see brauchen:

Berge brauchen Schnee. – Mountains need snow.
But here Bergsteigen is one activity, so braucht is correct.

What is the function of viel in viel Ausdauer? Why not an article like eine Ausdauer?

Ausdauer (stamina, endurance) is an uncountable, abstract noun in this context. You don’t say one stamina in English; similarly, you don’t say eine Ausdauer in German.

Instead, you use viel (much / a lot of) to express an indefinite large amount:

viel Ausdauer – a lot of stamina
viel Geduld – a lot of patience
viel Zeit – a lot of time

So viel is functioning like much / a lot of, not like an article. That’s why there is no article before Ausdauer in this sentence.

Why is there a comma before deshalb?

The sentence actually has two main clauses:

Bergsteigen braucht viel Ausdauer,
deshalb trainiere ich dreimal pro Woche.

In German, when you connect two independent main clauses, you normally put a comma between them, even if the second clause starts with an adverb like deshalb.

So the comma separates:

first statement: Mountain climbing requires a lot of stamina
consequence: therefore I train three times per week

Why does the word order after deshalb become deshalb trainiere ich and not deshalb ich trainiere?

German has a strict verb-second (V2) rule in main clauses: the finite verb (here: trainiere) must be in second position.

In the second clause:

deshalb takes the first position (it’s a connector / sentence adverb)
the verb (trainiere) must therefore come next, in second position
the subject (ich) comes after the verb

So the correct order is:

deshalb trainiere ich (1. deshalb – 2. trainiere – 3. ich)

deshalb ich trainiere would break the V2 rule and is incorrect in a main clause.

Is deshalb a conjunction like weil, or something else?

Deshalb is a sentence adverb (or Pronominaladverb), not a subordinating conjunction like weil.

deshalb introduces a new main clause and keeps the V2 word order:
- Bergsteigen braucht viel Ausdauer, deshalb trainiere ich dreimal pro Woche.
weil introduces a subordinate clause and sends the verb to the end:
- Bergsteigen braucht viel Ausdauer, weil ich dreimal pro Woche trainiere.

Meaning-wise they’re close, but grammatically:

deshalb = therefore / for that reason → main clause, V2 word order
weil = because → subordinate clause, verb at the end

Could I replace deshalb with deswegen or darum? Would the word order change?

Yes, you can replace deshalb here with deswegen or darum without changing the word order:

Bergsteigen braucht viel Ausdauer, deswegen trainiere ich dreimal pro Woche.
Bergsteigen braucht viel Ausdauer, darum trainiere ich dreimal pro Woche.

All three (deshalb, deswegen, darum) are sentence adverbs that:

introduce a main clause
take the first position
force the verb into second position

So the pattern stays:
deshalb / deswegen / darum + verb + subject + ...

Why is dreimal written as one word and not drei mal?

Dreimal is an adverb meaning three times, and as an adverb it is written as one word.

einmal – once
zweimal – twice
dreimal – three times
mehrmals – several times

You write drei Mal (two words) only when Mal is treated as a noun meaning occurrence/time and can take an article or adjective:

das erste Mal – the first time
ein Mal – one time
viele Male – many times

In your sentence, it’s an adverb of frequency → dreimal (one word).

What does pro Woche literally mean, and could I say something else instead?

Pro Woche literally means per week. It expresses a rate / frequency.

You could also say:

dreimal in der Woche – three times in the week
ich trainiere drei Mal die Woche (more colloquial)
ich trainiere dreimal wöchentlich (more formal)

Dreimal pro Woche is very common, neutral, and clear.

Why is the present tense trainiere used, not something like a future tense?

German uses the present tense much more often than English, even for regular future actions.

Ich trainiere dreimal pro Woche.
= I train three times a week. / I work out three times a week.
(This includes both present and general/future habit.)

If you really wanted to stress a future plan, you could say:

Ich werde dreimal pro Woche trainieren.

But for habits and regular activities, present tense is standard and natural.

Why is the subject ich placed after trainiere and not before, as in English?

Again, this follows the verb-second rule in main clauses.

In the second clause, the first position is taken by deshalb. That means the verb must be second, and everything else (including the subject ich) has to come afterward:

Position 1: deshalb
Position 2: trainiere (finite verb)
Then: ich (subject) + dreimal pro Woche (rest of the sentence)

If you wanted the subject first, you would simply drop deshalb or move it:

Ich trainiere deshalb dreimal pro Woche. (Subject first, deshalb in the middle)
But with deshalb at the beginning, ich must come after the verb.

Could I also say Bergsteigen erfordert viel Ausdauer instead of braucht? Is there a difference?

Yes, Bergsteigen erfordert viel Ausdauer is perfectly correct and quite natural.

Subtle differences:

brauchen = to need / require (more everyday, neutral)
erfordern = to require, to demand (a bit more formal / technical)

In everyday speech:

Bergsteigen braucht viel Ausdauer. – very common, conversational In slightly more formal style:
Bergsteigen erfordert viel Ausdauer.

Both are correct; the choice is mostly about style and tone.