1.I decided that I wouldn't talk to her ! Correct. Better: "I decided not to talk to her" 2.I decided that I wouldn't talk to her again! Correct, but it means you talked to her before, and will not to it again. Better: "I decided not to talk to her again" 3.I decided not to talk to her again! Perfect :) 4.I decided that I never talk to her! Incorrect. You must use a modal verb (فعل وجهی) after "decide that". 5.I decided that I would never ever talk to her again. Correct. Better: "I decided to never ever talk to her again". "Never ever" is informal. #4 is wrong. #1, 2, 5 are unnatural. When the "decide" and the main verb have the SAME subject, use "decide to do". If the two verbs have DIFFERENT subjects, use "decide [that]". You can remove "that". For example I decided to go (not "I decided that I will go") My friend decided to cook dinner tonight We decided to watch a movie But... I decided [that] you don't have to go My friend decided [that] I should cook dinner We decided [that] this movie is boring

They're all correct except for 4. 2,3 and 5 are the same, just phrased differently. 1 is slightly different in that there's no "again", which means the person may have never talked to her before.

They are all correct, with slightly differing meanings.