how to say “mysterious” in Hebrew
Another application of ס.ת.ר is in the expression for hideout or place or refuge – מִסְתּוֹר.
מסתור also conveniently serves as the basis for the Hebrew word for mysterious – מִסְתּוֹרִי in the masculine and מִסְתּוֹרִית in the feminine.
They entered a mysterious forest.
As for a mystery, the word is מִסְתּוֹרִין, or its synonym, תַּעֲלוּמָה.
Note that the root ס.ת.ר also bears the meaning contradiction. The word itself for contradiction is סְתִירָה.