Learn Hardcore Portuguese (Portugal): Quando salto sem cuidado, sinto dor no tornozelo e perco o equilíbrio. - When I jump carelessly, I feel pain in my ankle and lose my balance.

salto, sinto, perco are all 1st person singular forms, so they already mean “I jump / I feel / I lose”.
You add eu only for emphasis or contrast, e.g. Eu salto, mas ele não salta. – I jump, but he doesn’t.

So Quando salto… is the normal, neutral way to say “When I jump…”.

Why is the present tense used in Quando salto sem cuidado if this seems like a repeated or general situation?

In Portuguese, the simple present is used for:

Habits and general truths:
- Quando salto sem cuidado, sinto dor… = Whenever I jump carelessly, I feel pain…
Also for future after time words like quando, se, logo que:
- Quando chegar, ligo-te. – When I arrive, I’ll call you.

Here, Quando salto… means “whenever I (normally) jump like that”, not just one single occasion.

Could I say Se salto sem cuidado instead of Quando salto sem cuidado? Is there a difference?

You can hear Se salto sem cuidado…, but quando is much more natural here.

Quando salto sem cuidado… → describes a typical pattern: Whenever I jump like this, X happens.
Se salto sem cuidado… → feels more like a conditional: If I (happen to) jump like this, then X might happen.

In European Portuguese, for this kind of habitual cause–effect, quando is the default choice.

What exactly does sem cuidado mean here? Could I say descuidado or descuidadamente instead?

sem cuidado literally means “without care / without caution”, so in English carelessly.

Alternatives:

de forma descuidada – in a careless way (slightly more formal)
de maneira imprudente – in a reckless / imprudent way
descuidadamente – grammatically correct, but in everyday European Portuguese it sounds a bit bookish.

The simplest and most natural for speech is sem cuidado or sem grande cuidado.

Is sinto dor no tornozelo the most natural way to say “I feel pain in my ankle” in European Portuguese?

It’s correct, but other options are often more idiomatic in Portugal:

Common ways to talk about pain:

Dói-me o tornozelo. – My ankle hurts. (very natural)
Tenho dores no tornozelo. – I have pain(s) in my ankle.
Sinto dor no tornozelo. – correct, a bit more neutral/formal or descriptive.

So your sentence is fine grammatically, but a very natural spoken version might be:

Quando salto sem cuidado, dói-me o tornozelo e perco o equilíbrio.

Why is dor singular here and not dores? What’s the difference between dor and dores?

Both are used, but there’s a nuance:

dor (singular)
- Focus on the sensation itself, often one specific pain:
  - Sinto uma dor aguda no tornozelo. – I feel a sharp pain in my ankle.
dores (plural)
- Very common in everyday speech for aches / pains in a more general or ongoing sense:
  - Tenho dores no tornozelo. – I have ankle pain / my ankle is hurting.

In your sentence, sinto dor no tornozelo = I feel pain in my ankle.
If you said sinto dores no tornozelo, it would sound a bit more like ongoing / recurrent pains.

What does no in no tornozelo mean exactly?

no is a contraction:

em (in, on, at) + o (the, masculine singular) → no

So:

no tornozelo = em + o tornozelo = in/on the ankle
Similarly: na perna = in/on the leg (em + a)

Here, dor no tornozelo literally is pain in the ankle.

Why is it no tornozelo and not no meu tornozelo when we’re clearly talking about my ankle?

In Portuguese, with body parts and clothing, you usually use a definite article instead of a possessive when it’s obvious whose body part it is:

Dói-me a cabeça. – My head hurts.
Lavei as mãos. – I washed my hands.
Magoei o tornozelo. – I hurt my ankle.

You only add meu / minha / meus / minhas when you need to emphasize or contrast:

Dói-me o meu tornozelo esquerdo, não o direito. – It’s my left ankle that hurts, not the right one.

So dor no tornozelo naturally means pain in my ankle here.

Why is it perco o equilíbrio and not just perco equilíbrio?

In Portuguese, abstract nouns like equilíbrio usually take an article in this kind of structure:

perder o equilíbrio – to lose (one’s) balance
perder a paciência – to lose patience
ter a certeza – to be sure

Without the article (perco equilíbrio), it sounds incomplete or ungrammatical in this context. The idiomatic expression is perder o equilíbrio.

Could I say desequilíbrio-me instead of perco o equilíbrio?

You’d say desequilíbro-me (present: desequilibrar-se → desequilíbro-me), and yes, it’s possible:

…e perco o equilíbrio.
…e desequilíbro-me.

Both are correct. Nuance:

perco o equilíbrio – very common, neutral, idiomatic.
desequilíbro-me – a bit more “technical” or descriptive, literally I unbalance myself.

In everyday speech, perco o equilíbrio is more frequent.

Can I move Quando salto sem cuidado to the end: Sinto dor no tornozelo e perco o equilíbrio quando salto sem cuidado?

Yes. Both orders are correct and natural:

Quando salto sem cuidado, sinto dor no tornozelo e perco o equilíbrio.
Sinto dor no tornozelo e perco o equilíbrio quando salto sem cuidado.

The difference is only in focus:

At the beginning, Quando salto sem cuidado highlights the condition / situation first.
At the end, you first present the consequences, then explain when they happen.

Could I use pulo instead of salto in European Portuguese?

In Portugal, saltar is the more common verb for “to jump”:

Quando salto sem cuidado… – very natural in Portugal.

pular / pulo is well understood, but in European Portuguese it’s:

less common in everyday speech
sometimes felt as more Brazilian-sounding in this sense.

So if you’re aiming for Portuguese from Portugal, salto is the best choice here.