Documented in Volume 1 of the UNIX Programmers Manual.
The explain command explains grammatical phrases that the command
diction has flagged as ungrammatical. End with a CTRL-D.
Use it like this, (where you type in the offending phrase at the
prompt 'phrase?'):
% explain
phrase?
in order to
use "to" for "in order to"
phrase?
finalize
use "complete, finish, end" for "finalize"
phrase?
utilize
use "use" for "utilize"
phrase?
which
use ""that" when clause is restrictive" for "which"
use "when" for "at which time"
phrase?
CTRL-D
%