Learn Polish: Nie zapomnij, że cię kocham! - Don't forget that I love you!

The ending -ij in zapomnij is a common ending for the singular informal "you" command. If you were telling a group of people not to forget, you would add -cie to the end to make it Nie zapomnijcie.

Why does the sentence use "cię" instead of "ciebie" for "you"?

cię is the short, unstressed form of the pronoun, while ciebie is the long form. In Polish, you almost always use the short form in the middle of a sentence unless you want to put special emphasis on the word "you".

Is the comma before "że" required?

Yes! Unlike in English where you can smoothly write "Don't forget that..." without a comma, Polish punctuation rules strictly require a comma before the conjunction że (that) when it connects two parts of a sentence.

Where is the word for "I" in "I love you"?

The verb kocham already contains the information that "I" am doing the action because of its -am ending. In Polish, subject pronouns like ja (I) are usually dropped unless you are specifically stressing them for contrast.