This is the 705th most frequent Finnish word.
tätä
The Finnish word "tätä" means "this" in the partitive case.
In this sentence, 'tätä' (in the form 'tästä') is used to indicate something specific that is being liked, with the verb 'pitää'.
Pidän tästä kirjasta.
I like this book.
'Tätä' modifies 'this project' in the context of an activity that is being discussed.
Kenen kanssa toimit tässä projektissa?
With whom are you working on this project?
In this question, the word 'tämä' (related form of 'tätä') identifies a specific, yet undefined, object or concept.