PSA Insert Text to SRT is a reliable tool that allows you to insert custom pieces of text in subtitle files, at clear moments. The application can read the imported files, in order to check the moments of text display. It can indicate whether or not the time you selected is already assigned to a particular line....
If you ever tried to do synchronize your video files with their subtitles, you know it's a hard and long thing to do. PSA Media & Subtitle Renamer is a free software that easily does the job in matter of seconds. There are a lots of tools built-in so there'd be nothing else needed.