Answer:Mr. Hale says that there is blood on his head because he feels responsible for the deaths of the innocents who have been hanged prior to Act Four as well as for those folks scheduled to hang in the final act of the play.
IF I can sing well enough, I'll join the school chorus. The IF implies that something is conditional i.e. it may happen, it may not happen (it depends on whether you can sing well enough!) Hope this helps... :)