Each of the words an, am, at, come, rate and many others can be transformed into a new valid word by adding a prefix be.
The following list consists of 16 words, which may be sorted into 4 groups, each of 4 words fitting the same two-letter prefix, specific for that group.
One such group needs KN as a prefix.
Other 2-letter prefixes are up to you to find.
All 16 words must be used.

1. able
2. aid
3. air
4. and
5. ant
6. art
7. ash
8. ave
9. easy
10. eel
11. idle
12. irk
13. other
14. out
15. own
16. ows

(In reply to with computer assistance by Charlie)

a. Your solution meets the requirements.

b. However one can avoid use of two related words in the KN group.

c. You can overcome this obstacle by yourself, - or wait couple of days till I post my original classification.

Posted by Ady TZIDON on 2016-08-16 14:35:25