If you have to insert a comment on several lines do you do manually insert the comment character in every line? Stop. Vim is a good editor and has a nice feature to accomplish this quickly. Here are the steps:

1. Enter visual blocking mode by pressing CTRL V (CTRL key and the lowercase v).
2. Make your selection using motion keys(jklm, etc.).
3. Press I(uppercase I) to enter block insert mode.
4. Press #, the comment character.
5. Press Esc key. The comment character # will be inserted on each line the visual block selection.

Read more about visual blocking mode using the vim help topic visual-block. At the command line(:), type help visual-block.