To merge one Subtitle to another, there are two methods:
1. Use Keyboard Shortcuts:
Merge selected titles to the next title/s:
Default Shortcut: CTRL+F7 / CTRL + m
This will merge the active subtitle with the next or selected subtitles.
Merge title with previous title:
Default Shortcut: CTRL+ ALT + F7 / CTRL + ALT + m
This will merge the active title with the previous one subtitle.
2. Via the Timeline
You can Right-Click on a subtitle directly on the Timeline Module to display an additional menu.
Select either Merge with Next, or Merge with Previous: