Learn Hardcore Czech: Myslím, že chyba není problém, když se chci učit. - I think that a mistake is not a problem when I want to learn.

chyba – a/the mistake; a mistake in general
Ta chyba není problém. – That mistake is not a problem.
Chyba není problém. – A mistake is not a problem / Mistakes aren’t a problem (in general).

Here, chyba is used in a general sense: a mistake / making a mistake is not a problem.

Why is it chyba není problém and not chyba není problémem?

Both are grammatically correct, but they feel slightly different:

Chyba není problém. – most common, neutral: A mistake is not a problem.
Chyba není problémem. – more formal, a bit more “logical” or contrastive: A mistake is not (what we would call) a problem.

In everyday speech, Czechs strongly prefer není problém here, using the nominative (problém), not the instrumental (problémem).

Why is it myslím and not myslím si? What is the difference?

Both are possible:

Myslím, že… – I think that…
Myslím si, že… – also I think that…

Differences:

myslím is slightly shorter and more neutral; very common in speech.
myslím si can sound a bit more personal (this is my own opinion), or just a stylistic variant. In many contexts they’re interchangeable.

So you could also say:

Myslím si, že chyba není problém, když se chci učit.

Why is there a comma before že and když?

In Czech, subordinate clauses are normally separated by commas.

že introduces a content clause (what you think):
- Myslím, že chyba není problém…
když introduces a time/condition clause:
- …, když se chci učit.

So commas mark the boundaries:

Myslím, že chyba není problém, když se chci učit.

In English you can drop some commas, but in Czech these commas are standard and expected.

Could I say Myslím, že není problém chyba, když se chci učit?

That word order is not natural.

In Czech, the default for such an X is not Y sentence is:

Chyba není problém. – A mistake is not a problem.

Reversing it to Není problém chyba would be unusual or confusing; it might sound like you are listing problém chyba as some fixed phrase.

You can, however, play with emphasis a bit:

Myslím, že problém není chyba, ale nedostatek snahy.
I think the problem is not the mistake, but the lack of effort.

Here problém není chyba is allowed because of a different focus. But in your original sentence, chyba není problém is the natural order.

What exactly does když mean here? Is it “when” or “if”? Could I use jestli or kdy?

Here, když means roughly “when(ever)”, with a bit of an “if” feeling:

…, když se chci učit.
…, when I want to learn / whenever I want to learn / if I want to learn.

Alternatives:

jestli – mostly if/whether (for conditions or questions):
- Není problém, jestli se chci učit. – sounds odd here.
kdy – means when in questions:
- Kdy se chceš učit? – When do you want to learn?

So in this sentence, když is the correct and natural choice; jestli and kdy would not work.

Why is it učit se and not just učit? Don’t both mean “to learn”?

Czech distinguishes teach and learn clearly:

učit = to teach (someone)
- učit děti angličtinu – to teach children English
učit se = to learn, to study (yourself)
- učit se česky – to learn Czech

The se is reflexive and shows that the action is directed back at the subject: I am learning myself.

So:

chci učit – I want to teach (someone).
chci se učit – I want to learn.

Your sentence must use se, because you mean learning, not teaching.

Can I change the position of se in když se chci učit?

In practice, no – not freely. Some examples:

✅ když se chci učit – completely natural.
❌ když chci se učit – sounds wrong to a native speaker.
❌ když chci učit se – also wrong.

Clitics like se usually stand in the second position in the clause, after the first stressed word or phrase:

Když se chci učit, vypnu telefon.
Já se chci učit.
Dnes se chci učit.

So když se chci učit is the correct and standard order.

Why is chyba singular if the idea is more general, like “mistakes aren’t a problem”?

Czech often uses the singular to express a general statement:

Chyba není problém. – literally A mistake is not a problem, but it can mean Mistakes are not a problem (in general).
Člověk je omylný. – literally Man is fallible, but means People are fallible.

You can also use plural:

Chyby nejsou problém, když se chci učit. – Mistakes are not a problem when I want to learn.

Both are correct; the singular sounds a bit more like “the fact of making a mistake” is not a problem.

Why is there no já for “I”? Shouldn’t it be Já myslím?

Czech usually omits subject pronouns because the verb ending already shows the person:

myslím = I think (1st person singular)
myslíš = you think (2nd person singular)
myslí = he/she/they think

So Myslím, že… is normally enough.

You add já for emphasis or contrast:

Já myslím, že chyba není problém.
I (personally) think that a mistake is not a problem.
(implied: maybe others don’t think so)

In neutral statements, you just say Myslím, že….