忙しい is the regular form, the が is being used here with something called okurigana. It is being used to disambiguate kanji as it has multiple readings. 忙しい can actually be read both いそがしい and せわしい however いそがしい is the common reading. I don't know much more then that but yes as Tomoko said, use 忙しい. Further reading: http://en.wikipedia.org/wiki/Okurigana#Disambiguation_of_kanji http://tangorin.com/general/%E5%BF%99%E3%81%97%E3%81%84

忙しい　　correct 忙がしい　　not correct You should use "忙しい".