If you have to edit a subtitle file directly in a text editor, do not use Windows Notepad or office software like Word or LibreOffice Writer.
Files edited with Windows Notepad will often lose line breaks and save with the wrong character encoding (ISO-8859-1 instead of the correct UTF-8).
Files edited with text processors may not preserve the correct format, and autocorrections may affect hyphens, quotation marks, etc. which should remain unaltered for the format to work properly.
If you have to edit a subtitle file directly, use a text editor intended for editing software code, e.g.: Notepad++ (Windows), BBEdit (Mac), or gedit (Linux).