all above sentences are grammatically correct. but the follow forms are the most often used and more natural. It is thoughtful of you to do me a favor. It is cunning of you to scheme my money. it may also used expressed as: How thoughtful you were to do me a favor. How cunning you were to scheme my money.

You are thoughtful to do me a favor. (NO) It is thoughtful of you to do me a favor. (YES) You are cunning to scheme my money. (NO) It is cunning of you to scheme my money. ("scheme my money" is bad grammar - no meaning) Are the all sentences above correct?! THEY have confused me for a long while. By the way, How "cordially" the "students" greet us. IS IT grammatically correct? (YES) You are trying to use adjectives incorrectly. Adjectives describe NOUNS and are NOT normally associated with verbs.