Haastaa is an old fashined word meaning “to talk” or “to chat”. It’s rarely used in it’s original meaning, but it’s still included in another verb “haastatella” = “to interview” (lit. ~ “make someone to chat” ).
“Toimittaja haastatteli poliitikkoa.” “The journalist interviewed the politician.”
In modern Finnish, “haastaa” has a spesific meaning “to challenge”, for example “Haastan sinut kaksintaisteluun!” “I challenge you to a duel!”
“Haastava” is an adjective and it means “challenging”, for example, “Kisa oli hyvin haastava” “The race was very challenging.”. The noun derived from “haastaa” is “haaste“, “a challenge”. “Otan kaikki vaikeudet haasteena” / “I will take all the difficulties as a challenge”.
In Finnish, to “sue somebody” is literarily “to challenge somebody into court” = “haastaa oikeuteen“. (see the article for “oikeus“)