ls

Firstly, your ls is probably aliased to something.

Check it with:

1alias ls

If the prompt shows some alias, then start by removing it:

1unalias ls

Now we can begin.

Check the most recently modified file:

1ls -t

Reverse this with tac to see the file which has been unmodified the longest:

1ls -t | tac

Group files by extension:

1ls -X

Sort largest files first:

1ls -X