These work for matching parenthesis (), square brackets [], curly brackets {}, angle brackets <>, etc…

y% - yank
d% - delete

• :help y
• :help d

For visual mode

v%y - highlight between parenthesis and yank
v%d - highlight between parenthesis and delete

With the cursor within a block

yab - yank the block including the enclosing parenthesis
dab - delete the block including the enclosing parenthesis
yib - yank the block excluding the enclosing parenthesis
dib - delete the block excluding the enclosing parenthesis

For visual mode

v2a) - visual select, 2 nesting levels, including the enclosing parenthesis
v1i) - visual select, 1 nesting level, excluding the enclosing parenthesis

• :help object-select

1. Enter visual mode: v
2. Select the lines to comment.
3. Add the line comment characters to the beginning of each line:

:s/^/#    - shell scripts
:s/^/\/\/ - C, C++, C#, Java, JavaScript
:s/^/--   - SQL

1. Enter visual mode: v
2. Select the lines to comment.
3. Remove the line comment characters to the beginning of each line:

:s/^#/    - shell scripts
:s/^\/\// - C, C++, C#, Java, JavaScript
:s/^--/   - SQL

1. Enter visual mode: v
2. Select the lines to comment.
3. > to increase indent one tab < to decrease indent one tab.
4. n> to increase indent n tabs.
5. n< to decrease indent n tabs.