We have found one software suitable in our database for this conversion.
You can simply rename the extension fom subtitle files (.sub) and change it to txt should you wish it, which is actually not sub to txt conversion, but make the subtitle file easily openeable using any text editor.
However, text editors can open text files regardless of their actual file extension with the File ► Open dialog window, so you actually do not even need to rename sub to txt.
SUB
Files with sub extension are usually video subtitles in one of the most common subtitle format. It's a plain text file format that contain individual text entries with time stamps that basically tell the media player which text and at what playback time it should show.
TXT
A txt file extension is related to the TXT file format used for subtitles. TXT subtitles are used by various programs, like Captions, DVD Junior, TMPlayer, etc.