to a female = tismekhee (תשמחי)to a male = tismakh (תשמח)to a group = tismekhoo (תשמחו)
to a female: at smekha?to a male: atah same'akh?
It depends on the situation and plurality, but generally: שמח (Sah-may-ach).
masculine: me'od same'ach (מאוד שמח)feminine: me'od smecha (מאוד שמחה)
Yom Huledet Sameach = יום הולדת שמח