You can use a verb stem plus たいです to describe your hope or aspiration. The combination of a verb plus たい conjugates as an い-adjective. たいですsentences are usually not used to report others wishes or aspirations.
I want to see a film this weekend.
I want to go to China someday.
To describe your observation to the effect that somebody wants to do something, you must use a special verb たがっているinstead of たい. This derives from the dictionary form たがる and indicates "I think that she wants to, because of the way she is behaving."
(It seems) Mary wants to drunk coffee.