Questions / Dubbi

Hello. I am learning Italian and was hoping someone can help me with a few minor questions I have. 

Which of the following is correct (and why)? 

"A dire la verita'" or "Per dire la verita'" ? 

"Si puo fare le correzioni" or "Si possono fare le correzioni"?

"del ospedale" or "dell'ospedale"?

Thanks!