How do I create an insert-mode abbreviation that expands a short sequence into longer text?

Explanation

The :iabbrev command defines insert-mode abbreviations — short character sequences that automatically expand into longer text when you type a non-keyword character (space, newline, punctuation) immediately after them. This is one of Vim's most underused features for reducing repetitive typing and fixing common typos on the fly.

How it works

• :iabbrev registers an abbreviation active only in Insert mode (unlike :abbreviate, which also applies to Command mode)
• When the {lhs} is followed by a non-keyword character, Vim replaces the typed text with {rhs}
• The trigger character is kept, so typing teh replaces teh with the and leaves the space in place
• Put abbreviations in your vimrc for permanent effect

Example

" Fix common typos
:iabbrev teh the
:iabbrev adn and
:iabbrev dont don't

" Expand code snippets
:iabbrev ifmain if __name__ == '__main__':
:iabbrev pdb import pdb; pdb.set_trace()

In a Python file, typing ifmain<CR> inserts:

if __name__ == '__main__':

Tips

• Use :iabclear to remove all insert abbreviations, or :iunabbrev {lhs} to remove a specific one
• View all active abbreviations with :iabbrev (no arguments)
• For multi-line expansions, use <CR> in the rhs: :iabbrev lorem Lorem ipsum dolor<CR>sit amet
• Abbreviations with the same lhs as a word in your text won't expand unless the lhs appears as a standalone sequence
• Use :iabbrev <expr> {lhs} {expr} for dynamic expansions like inserting the current date