Normally if you're in a certain directory you could do:
find *.txt
and it will work as expected, but it won't work recursively through child directories, here's the correct way to do it:
find . -type f -name *.txt
The "-type f" is optional because that means only files, but we could have specified d for directory etc...
The above command will work recursively as you'd expect. In that way I find "find" to be unintuitive but this is basically what all find functions on all computers are implemented AFAIK (eg. front end search GUI's for different OS's).
linux, recursively, extensionsnormally, directory, txt, directories, quot, optional, specified, etc, unintuitive, functions, implemented, afaik, eg, gui, os,