(もし)Sentenceとしたら、
Usage
Means assume/suppose that S. Thus, Sとしたら literally means "if I/we/you suppose that S".
Examples
もしタイムマシーンがあるとしたら、どの時代に行って、誰に会ってみたいですか。
If you had a Time Machine in which period would you go and who would you like to meet.
これらの辞書の中で、もしどれか一つを選ぶとしたら、やはり電子辞書にするだろう。
Among these dictionaries, if I had to choose one, I would still go for the electronic dictionary.