Sorry I can't answer in Mandarin. うんどうでもしたら　どうですか？ しis from verb （うんどう）する . Verb+たら means "if ~" . So the entire sentence means "What about if you do some exercise?" In more natural English, "Why don't you do some exercise?" 半分ぐらいしか　わかりませんでした。 ~しか + negative add the speaker's emotion that "~" was far less than his/her satisfaction or expectation, or emphasize the limitation. The obvious fact(without emotion) is he/she understood about half. "半分ぐらい　わかりました"