Find

To find a file changed in the last X days

The following prints a file changed in the last day

find . -ctime -1 -print

Also see ctime.

To find a file accessed 30 days or before

To see files that were accessed last 30 or more days from now you'd do

find . -atime +30 -print

Also see atime

Large files

find . -name '*' -size +500M

will search from the directory you're in, recursively, listing all files over 500M in size