Och, znam tę piosenkę!

In Polish, the subject pronoun is often omitted because the verb ending already tells you who is doing the action.

• znam = I know
• The ending -am here shows 1st person singular

So Ja znam tę piosenkę! is possible, but ja is usually unnecessary unless you want emphasis, contrast, or extra clarity.

• Znam tę piosenkę! = neutral, natural
• Ja znam tę piosenkę! = I know this song! / I know it

Znam is the 1st person singular present tense form of znać, which means to know, to be familiar with, or to know someone/something.

Examples:

• Znam Annę. = I know Anna.
• Znam ten film. = I know this film.
• Znam tę piosenkę. = I know this song.

With songs, films, books, places, or people, znać usually means to be familiar with.

Because piosenkę is the direct object of the verb znam, so Polish uses the accusative case here.

The feminine singular demonstrative ta changes like this:

• ta = nominative, meaning this when the noun is the subject
• tę = accusative, used for a direct object

So:

• Ta piosenka jest dobra. = This song is good.
  Here ta piosenka is the subject.
• Znam tę piosenkę. = I know this song.
  Here tę piosenkę is the object.

For the same reason: piosenka is in the accusative singular, not the nominative.

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

• nominative: piosenka = song
• accusative: piosenkę = song, as a direct object

So:

• Ta piosenka jest stara. = This song is old.
• Lubię tę piosenkę. = I like this song.
• Znam tę piosenkę. = I know this song.

Because in Polish, words that go together often have to match in case, gender, and number.

Here:

• piosenka is feminine singular
• it is the direct object
• so it goes into the accusative
• the demonstrative ta must also change to match it

That gives:

• ta piosenka = nominative
• tę piosenkę = accusative

This is a very common pattern in Polish.

This is one of the most important distinctions in Polish.

• znać = to know someone/something, to be familiar with
• wiedzieć = to know a fact, to know information

So in this sentence, znam is correct because a song is something you are familiar with.

Compare:

• Znam tę piosenkę. = I know this song.
• Wiem, że ta piosenka jest stara. = I know that this song is old.

You would not normally say wiem tę piosenkę.

Och is an interjection, similar to English Oh.

It adds emotion or reaction. In this sentence, it suggests something like:

• sudden recognition
• pleasant surprise
• excitement

So Och, znam tę piosenkę! feels like:

• Oh, I know this song!
• Oh, I recognize this song!

It is natural, though in everyday speech people might also simply say:

• O, znam tę piosenkę!
• Znam tę piosenkę!

A rough guide is:

Okh, znam tem pyo-SEN-ke!

A few key points:

• Och has the Polish ch sound, like the ch in Scottish loch or German Bach
• znam begins with a real zn- cluster
• tę before p is often pronounced close to tem
• piosenkę has stress on the penultimate syllable: pio-SEN-kę

Also, Polish ę is a nasal vowel, but in everyday pronunciation it often sounds less strongly nasal than learners expect, especially at the end of a word.

Yes. Polish word order is more flexible than English, because case endings show the grammatical role.

The most neutral version is:

• Znam tę piosenkę!

But other orders are possible for emphasis:

• Tę piosenkę znam! = This song, I know!
• Och, tę piosenkę znam! = Oh, this song I know!

The sentence you were given is natural and straightforward. Changing the order usually changes the focus or emphasis, not the basic meaning.

Because many native speakers use tą in everyday speech where standard grammar expects tę.

Strictly speaking, in standard Polish:

• tę piosenkę = correct accusative
• tą piosenką = instrumental

However, in casual spoken Polish, tą piosenkę is very common. Learners should usually produce tę piosenkę, because that is the standard form and will always be accepted.

So the safest rule is:

• say tę piosenkę
• recognize that tą piosenkę is common colloquially