Learn Hardcore Czech: Myslím, že učit se jazyky je pro moji práci nejdůležitější. - I think that learning languages is the most important thing for my work.

Myslím, že… = I think that…
First clause: Myslím (main clause)
Second clause: že učit se jazyky je pro moji práci nejdůležitější (subordinate “that”-clause)

Because these are two separate clauses, Czech spelling rules require a comma between them: Myslím, že…

What is the function of že here? Could I leave it out?

že means “that” (in the sense of “I think that…”).

Myslím, že učit se jazyky je… = I think that learning languages is…

Unlike English, where you can often drop that (“I think learning languages is…”), in Czech you normally keep že.

✗ Myslím učit se jazyky je… is not natural; it sounds wrong to Czech speakers. You need že to introduce the subordinate clause.

Why is it učit se and not just učit?

učit se and učit are two different verbs:

učit se = to learn (literally “to teach oneself”)
učit (někoho něco) = to teach (somebody something)

So:

učit se jazyky = to learn languages
učit jazyky = to teach languages

In your sentence you’re talking about learning languages, so the reflexive form učit se is required.

Why is the word order učit se jazyky je… and not je učit se jazyky…?

In this sentence, učit se jazyky (learning languages) acts as the subject of the verb je (is):

Učit se jazyky = learning languages
je = is
nejdůležitější = (the) most important (thing)

This is a very natural pattern in Czech:

Učit se jazyky je těžké. = Learning languages is hard.
Učit se jazyky je zábavné. = Learning languages is fun.

You can say Myslím, že je učit se jazyky pro moji práci nejdůležitější, but the original order (učit se jazyky je…) is simpler and more neutral. Different word orders are used to change emphasis, but all are grammatically possible.

Why is it jazyky and not jazyků?

The verb učit se takes a direct object in the accusative (what you are learning):

učit se co? → jazyky (languages, accusative plural)

jazyky is the accusative plural of jazyk (language).
jazyků is the genitive plural, used in different contexts:

Učím se jazyky. = I am learning languages.
Učím se hodně jazyků. = I am learning many languages.
Kolik jazyků se učíš? = How many languages are you learning?

So učit se jazyky is the correct form for “learn languages” as a direct object.

Why is it pro moji práci? Which case is that?

The preposition pro (for) is always followed by the accusative case.

pro koho? pro co? → for whom? for what?

práce (work, job) is feminine:

Nominative: práce
Accusative: práci

So:

pro práci = for (the) work
pro moji práci = for my work

moji is the accusative feminine singular form of můj (my), agreeing with práci.

Why is it moji práci and not moje práce?

moje is not used in every case; it changes form just like práce does. For a feminine noun:

Nominative (who/what?): moje práce (my work – as the subject)
Accusative (for whom/what?): moji práci

Because pro requires the accusative, you must say:

pro moji práci = for my work

You could also hear pro mou práci, where mou is a shorter, slightly more formal/“bookish” variant of moji in this case.

Could I say pro svou práci instead of pro moji práci?

Yes, and many teachers would even prefer it.

svůj / své / svou… is the reflexive possessive (“one’s own”).
When the possessor is the subject (já in Myslím…), standard Czech style often prefers svůj over můj.

So:

Myslím, že učit se jazyky je pro moji práci nejdůležitější.
Myslím, že učit se jazyky je pro svou práci nejdůležitější.

Both are correct and mean the same; pro svou práci just sounds a bit more “textbook correct” or careful.

What exactly does nejdůležitější mean and how is it formed?

nejdůležitější = the most important.

It comes from the adjective důležitý (important):

Positive: důležitý – important
Comparative: důležitější – more important
Superlative: nejdůležitější – most important

You form the superlative by adding the prefix nej- to the comparative:

důležitý → důležitější → nejdůležitější

In the sentence, nejdůležitější is a predicate adjective (saying what something is), so it appears in this general -í form, agreeing with the implied neuter to (see the next question).

Why doesn’t nejdůležitější agree with a clear noun? What is it agreeing with?

The structure is essentially:

(To) učit se jazyky je (to) pro moji práci nejdůležitější.

Czech often has an implied neuter pronoun “to” in such sentences. You could also say:

Myslím, že učit se jazyky je pro moji práci to nejdůležitější.

Here to nejdůležitější = the most important thing.

Even when to is not written, the predicate adjective (nejdůležitější) is still effectively agreeing with that neuter “to”, so it appears in the neuter-like form -í.

Could I move the phrase pro moji práci somewhere else in the sentence?

Yes. Czech word order is relatively flexible, and you can change the position to shift emphasis. For example:

Myslím, že pro moji práci je nejdůležitější učit se jazyky.
– Emphasis a bit more on for my work, the most important is…
Myslím, že je pro moji práci nejdůležitější učit se jazyky.
– Similar meaning, slightly different rhythm.

All of these are grammatical; your original sentence is neutral and clear.

What is the difference between myslím and myslím si?

Both are widely used and often interchangeable in this context:

myslet si = to think, to hold an opinion (reflexive)
myslet (without si) is also used with the same meaning in Myslím, že…

So:

Myslím, že…
Myslím si, že…

In everyday speech, both sound natural; myslím si can feel very slightly more colloquial or personal (“I myself think…”), but in this sentence either is fine.

Why do you use že and not aby in this sentence?

Because you’re reporting what you think is true, not expressing a purpose or wish.

že = that (introduces a statement you believe is true)
- Myslím, že učit se jazyky je… = I think that learning languages is…
aby = so that, in order that, often with a verb of intention or purpose
- Učím se jazyky, aby to pomohlo mojí práci.
  = I learn languages so that it helps my work.

In your sentence you are not saying “I do X so that Y happens”, but “I think that Y is true”, so že is the only correct conjunction here.