Force user to insert a comment based on a RAG satus

Hi this is my first attempt at soemthing like this and am therfore a complte novice.
I have a spreadsheet with three tabs on each of the tabs there is a list of project information and an associated rag status column. What I would like to happen is that when a user selects R or A it forces them to put a comment in a comment box associated to the cell to the left. The comment needs to include the time and date of comment aswell as free text. If a user then wishes to update the comment the existing comment should remain with the new comment. If a user selects G the comments should be deleted after a warning.
Iwould then like to export the comments and the work title t a summary page per tab.
I know this seems a lot to ask and I hope it makes sense, Any help would be greatly appreciated.
Thanks