Kojarzę tę piosenkę.

In this sentence, kojarzę means something like:

• I recognize this song
• This song seems familiar to me
• I know this song by association / from hearing it before

The verb kojarzyć can mean to associate in a general sense, but in everyday Polish it is also often used to mean to recognize or to know vaguely from somewhere.

So Kojarzę tę piosenkę is not necessarily as strong as I know this song very well. It can suggest: I’ve heard it before or it rings a bell.

Because the verb kojarzyć takes a direct object, and direct objects in Polish often go in the accusative case.

The dictionary form is:

• ta piosenka = this song (nominative)

But in the sentence, this song is the object of the verb, so it changes to accusative:

• tę piosenkę

Both words change because they are feminine singular:

• ta → tę
• piosenka → piosenkę

So:

• Ta piosenka jest dobra. = This song is good.
• Kojarzę tę piosenkę. = I recognize this song.

Because piosenka is a feminine noun, and in the accusative singular, many feminine nouns ending in -a change to -ę.

So:

• nominative: piosenka
• accusative: piosenkę

This is a very common pattern:

• kawa → kawę
• książka → książkę
• dziewczyna → dziewczynę

Since piosenka is the direct object here, the accusative form is required.

Because in Polish, the verb ending already shows who the subject is.

kojarzę means I recognize / I associate, because the ending -ę marks 1st person singular: I.

So ja is optional:

• Kojarzę tę piosenkę.
• Ja kojarzę tę piosenkę.

Both are grammatical, but the version without ja is more natural in many contexts.

You usually add ja only for emphasis, contrast, or clarity:

• Ja kojarzę tę piosenkę, ale on nie.
  I recognize this song, but he doesn’t.

The dictionary form is kojarzyć.

Here is the relationship:

• kojarzyć = infinitive
• kojarzę = I associate / I recognize

A few present-tense forms:

• kojarzę = I recognize
• kojarzysz = you recognize
• kojarzy = he/she/it recognizes
• kojarzymy = we recognize
• kojarzycie = you all recognize
• kojarzą = they recognize

Kojarzyć is imperfective.

That makes sense here because the sentence describes a general present-state meaning: I recognize this song / this song is familiar to me.

A related perfective verb is often skojarzyć, but the meaning can shift depending on context. Very often:

• kojarzyć = to associate / recognize
• skojarzyć = to make a connection, to realize an association

For example:

• Nie kojarzę tej nazwy. = I don’t recognize that name.
• Teraz skojarzyłem! = Now I got it! / Now I made the connection!

So in your sentence, the imperfective kojarzę is the natural choice.

Yes, but it is not exactly the same.

• Kojarzę tę piosenkę = I recognize this song / it sounds familiar
• Znam tę piosenkę = I know this song

Znam usually sounds stronger. It suggests more solid familiarity. You may know the melody, lyrics, title, or have heard it many times.

Kojarzę can be weaker and more tentative, like:

• I know it from somewhere
• I’ve heard it before
• It rings a bell

So if you want to express vague recognition, kojarzę is a very good choice.

A rough pronunciation guide is:

• Kojarzę ≈ koh-YA-zheh
• tę ≈ ten or teh with nasal coloring
• piosenkę ≈ pyo-SEN-keh with nasal coloring at the end

A few useful points:

• j in Polish sounds like English y in yes
• rz here sounds like zh in measure
• pio- sounds like pyo-
• final ę is often not pronounced as a fully strong nasal vowel in everyday speech

So the whole sentence is approximately:

koh-YA-zheh ten pyo-SEN-keh

That is only an approximation, but it is close enough to help.

In careful standard grammar, the accusative singular feminine form of ta is tę:

• Kojarzę tę piosenkę.

However, in everyday spoken Polish, many native speakers say tą piosenkę. This is extremely common in conversation.

So:

• tę piosenkę = standard written / careful form
• tą piosenkę = very common in speech

For a learner, it is best to recognize both, but in formal writing and careful study, use tę piosenkę.

It can change.

The neutral version is:

• Kojarzę tę piosenkę.

But Polish word order is flexible, and different orders can change emphasis:

• Tę piosenkę kojarzę.
  = This song, I recognize.
  This puts more focus on this song.

• Piosenkę tę kojarzę.
  This sounds more literary or marked.

Because Polish uses cases, the roles of the words are still clear even when the order changes.

Yes. In everyday Polish, kojarzyć kogoś often means to recognize someone or to know who someone is.

For example:

• Kojarzysz go? = Do you recognize him? / Do you know who he is?
• Kojarzę ją z pracy. = I know her from work / I recognize her from work

So the verb is very common not only with things like songs, names, and places, but also with people.

Yes, very natural.

Kojarzę tę piosenkę sounds like something a native speaker could easily say when hearing a song and realizing it is familiar.

For example:

• someone turns on music
• you suddenly recognize it
• you say: Kojarzę tę piosenkę.

It is a common, conversational way to express recognition without sounding too formal or too strong.