This is the 27th most frequent Polish word.
tej
"Tej" is the feminine singular genitive or dative form of "ta," meaning "this" or "that" in English.
Here, 'tej' is used as a demonstrative adjective modifying the feminine noun 'książki' (book) in the genitive case.
Nie widziałem tej książki na półce.
I didn't see this book on the shelf.
In this sentence, 'tej' is the dative form of the demonstrative pronoun referring to the feminine noun 'dziewczynie' (girl).
Tej dziewczynie pomogliśmy wczoraj.
We helped this girl yesterday.
Within this context, 'tej' is again a demonstrative adjective specifying the feminine noun 'sukienki' (dress) in the genitive case.