"If he had not modified the rules, he would not have won." is correct. "Had (not) + <past participle of verb>" indicates that this action had happened earlier in the past, before a more recent past incident (in this case, "he would not have won") "I was not able to tell him the truth, I'm sorry." is correct. 'True' is an adjective while 'truth' is a noun. Hope this helps! * P.S.: Just want to point out gently that the structure of your question should be "Are these sentences correct?" instead of "Are correct these sentences?" This applies to all questions in English. Cheers!

your first sentence is correct.