Najlepiej pracuję rano.

Because najlepiej is an adverb, while najlepszy is an adjective.

• dobry = good
• lepszy = better
• najlepszy = best

These are adjective forms, used to describe nouns.

But in this sentence, best describes how I work, so Polish uses the adverb:

• dobrze = well
• lepiej = better
• najlepiej = best

So:

• Najlepiej pracuję rano. = I work best in the morning.

Not:

• *Najlepszy pracuję rano. — incorrect

Pracuję is the 1st person singular present tense of pracować (to work).

So it means:

• pracuję = I work / I am working

A few present-tense forms of pracować are:

• ja pracuję = I work
• ty pracujesz = you work
• on/ona pracuje = he/she works
• my pracujemy = we work
• wy pracujecie = you all work
• oni/one pracują = they work

The ending -ę is a very common marker for I in Polish verbs.

Because in Polish, the verb ending often already tells you who the subject is.

• pracuję already means I work
• so ja is not necessary

That is why Polish often drops subject pronouns unless they are needed for emphasis or contrast.

Compare:

• Pracuję rano. = I work in the morning.
• Ja pracuję rano. = I work in the morning. / As for me, I work in the morning.

So the version without ja is completely normal.

It can mean either, depending on context.

Polish present tense often covers both:

• I work
• I am working

In this sentence, because of najlepiej and rano, it usually sounds like a general habit or tendency:

• Najlepiej pracuję rano. = I work best in the morning.

So the most natural interpretation is not I am working best right now, but rather mornings are when I function best for work.

Because rano can function on its own as an adverb of time.

So:

• rano = in the morning / early in the morning

Polish does not need a preposition here.

Examples:

• Wstaję rano. = I get up in the morning / early.
• Najlepiej myślę rano. = I think best in the morning.

You can also say:

• nad ranem = toward dawn
• rankiem = in the morning
• o poranku = in the morning, at daybreak (more literary)

But rano is the most common everyday choice.

Yes. Polish word order is fairly flexible, although some versions sound more natural than others depending on emphasis.

The neutral version here is:

• Najlepiej pracuję rano.

Other possible versions:

• Rano pracuję najlepiej. = In the morning, I work best.
• Pracuję najlepiej rano. = I work best in the morning.
• Najlepiej rano pracuję. = possible, with a different rhythm/emphasis

The basic meaning stays similar, but the focus shifts:

• starting with najlepiej emphasizes best
• starting with rano emphasizes in the morning

Yes, very often. That can confuse learners.

Najlepiej can mean:

1. best

   • Najlepiej pracuję rano. = I work best in the morning.

2. best / preferably / the best thing would be

   • Najlepiej zadzwonić jutro. = It’s best to call tomorrow.
   • Najlepiej zostań w domu. = You’d better stay at home.

In your sentence, it clearly means best because it modifies the verb pracuję.

A rough pronunciation guide for an English speaker is:

• Najlepiej ≈ nai-LEP-yay
• pracuję ≈ pra-TSOO-yeh
• rano ≈ RA-no

A few useful pronunciation points:

• j in Polish sounds like English y in yes
• c sounds like ts
• ę at the end of pracuję is often pronounced close to e or ye in normal speech
• stress is usually on the second-to-last syllable:
  • naj-LE-piej
  • pra-CU-ję
  • RA-no

They are very similar, but rano is more common in everyday speech.

• rano = in the morning / early
• rankiem = in the morning

Examples:

• Pracuję rano.
• Pracuję rankiem.

Both are correct, but rano is usually the more natural default in conversation.

rankiem can sound a little more literary or stylistically marked in some contexts.

You add nie before the verb:

• Nie pracuję najlepiej rano.

But be careful: this sounds a little unusual if you mean the opposite of the original idea.

More natural alternatives might be:

• Nie pracuję najlepiej rano, tylko wieczorem.
  = I don’t work best in the morning, but in the evening.

• Rano nie pracuję dobrze.
  = I don’t work well in the morning.

So the best negative version depends on what exactly you want to deny:

• the best part
• the working well part
• or the morning part

Yes, but it is slightly different in tone.

• rano = in the morning
• z rana = in the morning, especially early in the morning

So:

• Najlepiej pracuję rano. = I work best in the morning.
• Najlepiej pracuję z rana. = I work best early in the morning.

z rana is perfectly natural, but it is a bit more specific.