A 'complete' word is a word in which every letter, in turn, can be replaced by another to form a new word.
For example, SEND is a complete word because:
Starting with SEND we can replace the S with an L to make LEND
Starting with SEND we can replace the E with an A to make SAND
Starting with SEND we can replace the N with an E to make SEED
Starting with SEND we can replace the D with a T to make SENT.
Which 2 of the following words are not complete words? Proper nouns (i.e. people's names) are not allowed.
HARD
PORT
DUCK
BUGS
BAIL
MAST
THIN
LEAN
FIRE
READ