Learn Hardcore German: Manchmal ist das Wetter zu schlecht, um draußen für die Ausdauer zu laufen, also trainiert sie drinnen. - Sometimes the weather is too bad to run outside for endurance, so she trains indoors.

All of these are grammatical; German just requires that the finite verb (ist) stays in second position in the clause, no matter what comes first. So:

Manchmal ist das Wetter … (adverb first, then verb)
Das Wetter ist manchmal … (subject first, then verb)

How does the structure zu schlecht, um draußen … zu laufen work? Is it like English “too bad to run”?

Yes, it’s very close to English too … to …. The pattern is:

zu + adjective, um … zu + infinitive

So:

zu schlecht, um draußen zu laufen
= too bad to run outside

Some key points:

zu schlecht = too bad
um … zu laufen is an infinitive clause (no finite verb, just laufen with zu).
Everything that belongs to the verb (like draußen für die Ausdauer) usually comes before the infinitive:
um draußen für die Ausdauer zu laufen

The comma before um is standard in this construction: it separates the main clause from the infinitive clause.

Could I also say so schlecht, dass sie nicht draußen laufen kann? What’s the difference from zu schlecht, um … zu laufen?

You can say that, and it’s very natural German, but there’s a nuance:

zu schlecht, um draußen zu laufen
focuses on the impossibility or unsuitability: the weather is too bad to run.
so schlecht, dass sie nicht draußen laufen kann
describes a result: it is so bad that she can’t run outside.

Important:

You cannot say *zu schlecht, dass … – zu goes with um … zu,
and so goes with dass:
- zu kalt, um zu baden
- so kalt, dass man nicht baden kann

Why is laufen used here and not rennen or joggen?

In sports/fitness contexts, laufen is the normal general verb for going running / jogging.

laufen – to run / go running (can cover jogging as a form of endurance training)
rennen – to sprint / run fast (often with a sense of hurry or speed)
joggen – to jog, specifically that type of running

In für die Ausdauer laufen, the focus is on endurance training, not on speed, so laufen is the usual choice. You could say joggen:

… um draußen für die Ausdauer zu joggen …

That would be understood as specifically doing jogging as a sport.

What exactly does für die Ausdauer mean, and why is it in that form?

Ausdauer means endurance / stamina and is grammatically feminine: die Ausdauer.

The preposition für always takes the accusative case, so you get:

für + die Ausdauer (accusative feminine singular)

Literally: for the endurance.
In natural English we’d say something like to build endurance or for stamina training:

… um draußen für die Ausdauer zu laufen …
≈ “… to run outside for endurance / for stamina.”

You can also rephrase in German, for example:

… um ihre Ausdauer zu trainieren. (to train her endurance)

What’s the difference between draußen / drinnen and außen / innen?

In this context you need draußen and drinnen:

draußen – outside / outdoors
- Ich laufe draußen. – I run outside.
drinnen – inside / indoors
- Ich trainiere drinnen. – I train indoors.

außen and innen are more about the inside/outside of an object or surface:

außen – on the outside (outer side)
- Die Tür ist außen rot. – The door is red on the outside.
innen – on the inside (inner side)
- Die Tasche ist innen blau. – The bag is blue on the inside.

So for indoors vs outdoors, use drinnen / draußen, not innen / außen.

Why is it das Wetter? What gender and case is that?

Wetter is neuter, so its definite article in the nominative singular is das:

das Wetter

In this sentence, das Wetter is the subject of the verb ist, so it must be in the nominative case:

Das Wetter ist zu schlecht …

There is no reliable rule that predicts this gender; you mostly have to learn it with the noun: das Wetter, das Klima, das Gewitter, etc.

What does also mean here? Is it the same as English also?

Here, also means roughly so / therefore, not also in the sense of too / as well.

… also trainiert sie drinnen.
≈ “… so / therefore she trains inside.”

For English also / too, German normally uses auch:

Sie läuft draußen, und sie trainiert auch drinnen.
– She runs outside, and she also trains indoors.

So be careful: German also usually corresponds to English “so / therefore”, not to English “also”.

Why is the word order also trainiert sie drinnen and not also sie trainiert drinnen?

In a main clause, German wants the finite verb in second position (the V2 rule).

In also trainiert sie drinnen:

also – first element (position 1)
trainiert – finite verb (position 2)
sie – subject
drinnen – adverb

So the order is correct: [also] [trainiert] [sie] [drinnen].

If you put sie after also:

*also sie trainiert drinnen

then trainiert would no longer be in second position, which breaks the V2 rule.

You can move also to the middle:

Sie trainiert also drinnen.

Here, sie is first, trainiert second, and also is just a mid-sentence adverb.

Who or what does sie refer to here? Could it be Sie (formal “you”)?

In this sentence, sie (lowercase) with the verb trainiert is:

3rd person singular feminine – she trains

If it were they, the verb would be plural:

Sie trainieren drinnen. – They train indoors. (note -en ending)

If it were formal you, it would be capitalized and also take the plural verb:

Sie trainieren drinnen. – You (formal) train indoors.

So here, because it’s lowercase and the verb is singular (trainiert), it clearly means she.

Why is trainiert in the simple present? Would German ever use a continuous form like “is training”?

Standard German doesn’t normally have a separate continuous tense like English is training. The Präsens (simple present) covers both:

sie trainiert
= she trains / she is training, depending on context.

With Manchmal (“sometimes”), it clearly expresses a habit:

Manchmal … trainiert sie drinnen.
– Sometimes … she trains indoors.

There is a colloquial construction ist am Trainieren, but it’s not needed here and is less neutral in style.