How to insert a block of white spaces starting at the cursor position in vi?

I'd use :%s/^/ /

You could also specify a range of lines :10,15s/^/ /

Or a relative range :.,+5s/^/ /

Or use regular expressions for the locations :/A/,/D/>.

For copying code to paste on SO, I usually use sed from the terminal sed 's/^/ /' filename

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Shortcut

I just learned a new trick for this. You enter visual mode v, select the region (with regular movement commands), then hit : which gives you this:

:'<,'>

ready for you to type just the command part of the above commands, the marks '< and '> being automatically set to the bounds of the visual selection.

To select and indent the current paragraph:

vip>

or

vip:>

followed by enter.

Edit:

As requested in the comments, you can also add spaces to the middle of a line using a regex quantifier \{n} on the any meta-character ..

:%s/^.\{14}/& /

This adds a space 14 chars from the left on each line. Of course % could be replaced by any of the above options for specifying the range of an ex command.

When on the first A, I'd go in block visual mode ctrl-v, select the lines you want to modify, press I (insert mode with capital i), and apply any changes I want for the first line. Leaving visual mode esc will apply all changes on the first line to all lines.

Probably not the most efficient on number of key-strokes, but gives you all the freedom you want before leaving visual mode. I don't like it when I have to specify by hand the line and column range in a regex command.