delete line in vi

Pressing dd will remove that line (actually it will cut it). So you can paste it via p.

As others said, it's dd if you're in normal mode (press ESC) to ensure you are in normal mode, but by default, you will be. You may have found that vi doesn't act as you would expect an editor to act. It's pretty unique, and those unique features make it extremely powerful and fast to work with. If you want to learn more, you can run vimtutor at the command line, and you'll get a quick (maybe an hour worth of reading/experimenting) tutorial on the basics of how to use it.

Now, you can use an editor with a more familiar interface. Most systems have nano. You can type nano a.txt to open your file in nano. You can even go a step further - there isn't any reason you have to use a terminal-based editor. Since you mentioned using (Ctrl+Alt++T) and this is the default shortcut in Ubuntu and some other Gnome distributions, so I'd be surprised if you didn't have gedit installed. At the command line, try gedit a.txt and you'll get a graphical editor.

To delete the line under the cursor, use dd.

The delete command accepts all the normal positional modifiers, so if you are on the beginning of the line below the one you want to delete, you could just dk to go into delete mode and move up one line, deleting everything the cursor passed.

You can also specify repetition, so 5dd will delete the line under the cursor five times, effectively deleting the current and following four lines.