Nie rozumiem tego słowa.

Because the verb already tells you the subject.

Rozumiem means I understand:

• rozum- = the verb stem
• -iem = 1st person singular present-tense ending

So Nie rozumiem tego słowa. already means I do not understand this word.

You can say Ja nie rozumiem tego słowa, but ja is usually added only for emphasis, contrast, or clarity.

Here is the breakdown:

• Nie = not
• rozumiem = I understand
• tego = a form of ten / to / ta meaning this or that, here in the genitive singular
• słowa = a form of słowo (word), here also genitive singular

So the sentence is literally something like:

Not understand-I of-this of-word

That sounds strange in English, but it is normal Polish grammar.

In Polish, nie is usually written separately with finite verbs.

So:

• nie rozumiem = I do not understand
• nie wiem = I do not know
• nie mam = I do not have

This is different from some other Polish words, where nie may be written together, especially with adjectives or adverbs:

• niedobry = bad / not good
• niedobrze = badly / not well

But with a verb like rozumiem, it stays separate.

Because after a negated verb, Polish often puts the direct object in the genitive instead of the accusative.

Compare:

• Rozumiem to słowo. = I understand this word.
• Nie rozumiem tego słowa. = I do not understand this word.

In the positive sentence:

• to słowo is the object

In the negative sentence:

• the object changes to genitive
• so to becomes tego
• and słowo becomes słowa

This is a very important and very common Polish pattern.

Here, słowa is genitive singular of słowo.

The basic forms are:

• słowo = nominative singular
• słowa = genitive singular

So in this sentence, słowa does not mean words. It means of the word / this word as required by the negation pattern.

A learner may notice that słowa can also be nominative plural (words), but here it cannot be plural, because tego shows it is singular.

Tego is a form of the demonstrative ten family, which means this (and sometimes that, depending on context).

Because słowo is neuter singular, the normal nominative/accusative form would be:

• to słowo = this word

But in this negative sentence, the noun phrase is in the genitive singular, so it becomes:

• tego słowa

So tego is simply the genitive singular form that matches the grammar of the sentence.

Because rozumieć is the dictionary form of the verb: to understand.

In a real sentence, you usually need a conjugated form:

• rozumieć = to understand
• rozumiem = I understand
• rozumiesz = you understand
• rozumie = he/she/it understands

So Nie rozumiem tego słowa uses the correct present-tense form for I.

Yes. Polish word order is more flexible than English, because endings show grammatical relationships.

The most neutral version is:

• Nie rozumiem tego słowa.

But you may also hear:

• Tego słowa nie rozumiem.

This puts more emphasis on this word.

So the meaning stays basically the same, but the focus changes. The original sentence is the most natural neutral version for a learner to use.

A rough guide is:

nyeh ro-ZOO-myem TEH-go SWO-va

More carefully:

• Nie sounds roughly like nye
• rozumiem has stress on zu: ro-ZOO-myem
• tego = TEH-go
• słowa = SWO-va because ł is pronounced like English w

The normal stress in Polish is usually on the second-to-last syllable:

• roZUmiem
• TEgo
• SŁOwa

Usually, not in the same way.

• rozumieć = to understand as a state
• zrozumieć = to come to understand / to understand completely

So:

• Nie rozumiem tego słowa. = I do not understand this word.

But zrozumieć would suggest the action of managing to understand something, not just the state of understanding. So for this sentence, rozumieć is the natural choice.