Nie umiem dobrze wymawiać tego słowa.

Umiem is the 1st person singular form of umieć.

In this pattern, umieć + infinitive means to know how to do something or to be able to do something because you have learned the skill.

So:

• umiem wymawiać = I know how to pronounce / I can pronounce
• nie umiem wymawiać = I don’t know how to pronounce / I can’t pronounce

This is about ability or learned skill, not just possibility.

They are similar, but not identical:

• nie umiem = I don’t know how / I’m not able because I lack the skill
• nie potrafię = very close to nie umiem; often a bit more emphatic, like I’m unable to
• nie mogę = I can’t because of circumstances, permission, or physical possibility

Examples:

• Nie umiem dobrze wymawiać tego słowa.
  I haven’t mastered pronouncing this word.

• Nie potrafię dobrze wymawiać tego słowa.
  Very similar; slightly stronger or more formal.

• Nie mogę dobrze wymawiać tego słowa, bo boli mnie gardło.
  I can’t pronounce this word well because my throat hurts.

So in your sentence, nie umiem is a very natural choice because it talks about skill.

Because after umieć, Polish normally uses the bare infinitive.

So the structure is:

• umiem + infinitive
• nie umiem + infinitive

Examples:

• Umiem pływać. = I can swim.
• Nie umiem gotować. = I can’t cook.
• Nie umiem dobrze wymawiać tego słowa.

Unlike English, Polish does not need a separate word like to before the infinitive here.

This is about aspect.

• wymawiać = imperfective
• wymówić = perfective

In this sentence, wymawiać is more natural because the speaker is talking about a general ability or repeated action: pronouncing a word correctly in general.

So:

• Nie umiem dobrze wymawiać tego słowa.
  = I’m not good at pronouncing this word / I don’t know how to pronounce it well.

A perfective form like wymówić would point more toward one completed attempt or successfully getting it out once.

Compare:

• Nie potrafię dobrze wymówić tego słowa.
  = I can’t manage to pronounce this word well.

That sentence is possible, but it feels a little more like a specific performance problem, while wymawiać is the best choice for general ability.

Because the object is in the genitive, not the accusative.

A major reason is negation: in standard Polish, the direct object of a negated verb often appears in the genitive.

Compare:

• Umiem dobrze wymawiać to słowo.
  affirmative

• Nie umiem dobrze wymawiać tego słowa.
  negative

So the forms change like this:

• to słowo = nominative/accusative
• tego słowa = genitive

This is a very common Polish pattern and often surprises English speakers, because English does not do anything similar.

It is genitive singular.

Here is the breakdown:

• słowo = word
• słowa = genitive singular of słowo
• to = this in nominative/accusative neuter singular
• tego = genitive singular form of ten / to

So:

• to słowo = this word
• tego słowa = of this word / this word in the genitive form

Even though słowo is neuter, the genitive form of the demonstrative is still tego.

Dobrze is an adverb meaning well. It modifies wymawiać.

So the sentence does not necessarily mean the speaker is completely unable to pronounce the word at all. It more naturally means:

• they do not pronounce it well
• they do not pronounce it correctly enough
• they are not confident with it

Compare:

• Nie umiem wymawiać tego słowa.
  I can’t pronounce this word.

• Nie umiem dobrze wymawiać tego słowa.
  I can’t pronounce this word well.

So dobrze softens the statement a bit and makes it more specific.

Yes. Polish word order is fairly flexible.

The version you have is the most neutral and natural:

• Nie umiem dobrze wymawiać tego słowa.

But other orders are possible, especially for emphasis:

• Tego słowa nie umiem dobrze wymawiać.
  Emphasizes this word in particular.

• Nie umiem tego słowa dobrze wymawiać.
  Also possible; slightly different rhythm.

The original sentence is a very good default choice for learners.

Yes. With verbs, nie is written separately.

So:

• nie umiem
• nie wymawiać
• nie mogę
• nie potrafię

This is standard Polish spelling.

So Nie umiem is correct, and Nieumiem would be wrong.

A rough pronunciation guide is:

Nie umiem dobrze wymawiać tego słowa
≈ Nyeh OO-myem DOB-zheh vuh-MAH-vyach TEH-go SWO-va

A more accurate IPA version is:

[ɲɛ ˈu.mjɛm ˈdɔ.bʐɛ vɨˈma.vjat͡ɕ ˈtɛ.ɡɔ ˈswɔ.va]

A few useful pronunciation notes:

• nie sounds roughly like nyeh
• rz in dobrze sounds like the s in measure, but with a more Polish quality
• wy does not sound like English why
• ć at the end of wymawiać is a soft consonant, somewhat like a very soft ch/tch

If you want, I can also break the whole sentence down word by word with pronunciation tips.