Korzeń gnije.

A careful pronunciation is:

• korzeń → KO-rzheń
• gnije → GNYEE-ye

More precisely:

• korzeń is roughly /ˈkɔ.ʐɛɲ/
• gnije is roughly /ˈɡɲi.jɛ/

A few important sounds:

• rz sounds like Polish ż: similar to the sound in English measure, but usually stronger.
• ń is a soft n, similar to Spanish ñ.
• gn in gnije is not like English gn in gnome. You pronounce both parts, with a hard g followed by a soft n sound.

The stress is on the second-to-last syllable, which here means:

• KO-rzeń
• GNI-je

Korzeń is:

• a noun
• singular
• nominative case
• masculine inanimate

It is in the nominative because it is the subject of the sentence: the thing that is rotting.

The dictionary form is also korzeń, so in this sentence you are seeing the basic form of the noun.

Because the subject korzeń is:

• third person
• singular

So the verb must also be third-person singular.

The verb is gnić → to rot. Its present-tense forms are:

• (ja) gniję — I rot / am rotting
• (ty) gnijesz — you rot / are rotting
• (on/ona/ono) gnije — he/she/it rots / is rotting
• (my) gnijemy
• (wy) gnijecie
• (oni/one) gniją

Since korzeń is singular, gnije is the correct form.

The dictionary form is gnić.

That is the infinitive, meaning to rot.

So:

• gnić — to rot
• gnije — rots / is rotting

This is a useful pattern to get used to in Polish: the form you see in a sentence is often different from the dictionary form.

Yes. Gnije is present tense.

And yes, in Polish the present tense often covers both ideas that English separates into:

• rots
• is rotting

So Korzeń gnije can match either of these in English, depending on context.

That is very normal in Polish. Polish does not require a separate form for the English-style present continuous.

Because Polish has no articles.

So korzeń can mean:

• a root
• the root

Which one is intended depends on context.

This is one of the biggest differences from English. In Polish, you usually do not add a separate word for definiteness or indefiniteness.

Yes, but Korzeń gnije is the most neutral, basic order.

This is:

• subject + verb

You may also see:

• Gnije korzeń

That is grammatically possible, but it sounds more marked. It may be used for emphasis, contrast, or a particular style.

So for a learner, Korzeń gnije is the safest and most natural default.

The -ń shows a soft n sound at the end of the word.

It is part of the normal spelling of the noun korzeń. You should not replace it with a plain n.

This softness matters in Polish pronunciation and sometimes also shows up in related forms. For example, when the word changes form, the stem may look slightly different:

• korzeń — nominative singular
• korzenia — genitive singular
• korzenie — nominative plural

So the word can change shape a bit when declined, which is very common in Polish nouns.

Gnić is imperfective.

That means it describes an ongoing process or a general situation:

• Korzeń gnije — The root is rotting / rots.

A common perfective partner is zgnić:

• Korzeń zgnije — The root will rot away / will become rotten.

This is a very useful Polish pattern:

• imperfective → process, repeated action, general action
• perfective → completed result, single whole event

So in your sentence, gnije focuses on the rotting as a process, not on its completion.

Yes. Like English root, Polish korzeń can be used in more than one sense.

Depending on context, it can refer to things like:

• a plant root
• the root of a problem
• a word root
• a mathematical root

In Korzeń gnije, the context strongly suggests a physical root, especially a plant root, because gnić means to rot. That makes the literal meaning the natural one here.