Car won't o into gear when started :(

Car won't o into gear when started :(,AI智能索引,全网链接索引,智能导航,网页索引

Car is off, goes into gears fine with no clutch and with clutch. Cars on, wont go into gear what so ever. Car rocks when attempting to go into gear with or without clutch pedal down. Slave cylinder just replaced because pedal went to the floor while driving with pressure on clutch pedal (to brake). Changed transmission fluid. Master cylinder looks near brand new. System holds pressure just fine, clutch pedal feels decent. HELP!