Vim

Copy all the lines to clipboard

y G

or

:%y

Cut the whole file

d G

Yank N lines

The usual mnemonic: {N}yy. For example, 10yy yanks the current line plus the next 9 lines (10 lines total).

From the current line downward

yy      # yank the current line
3yy     # yank 3 lines starting from the current line
5yy     # yank 5 lines starting from the current line

Pattern: {N}yy, where N is the number of lines.

In Vim, Y is a synonym for yy. Neovim maps Y to y$ by default (yank to end of line), so use yy to yank a whole line.

Yank to a line number

y5G     # yank from the current line to line 5
yG      # yank from the current line to the end of the file
ygg     # yank from the current line to the start of the file

Visual mode (most intuitive)

V       # enter linewise visual mode
j / k   # move the selection up or down
y       # yank

Or V, then 3j, then y: current line plus 3 lines down, 4 lines total.

Easy to mix up

yy      # yank the current line
y$      # yank from the cursor to the end of the line, not the whole line
y0      # yank from the cursor to the start of the line

Paste

p       # paste after the cursor / on the next line
P       # paste before the cursor / on the previous line