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 first thoughts by Charlie)

The given in the puzzle is also an overabundance:

A knout is a whip that was used to inflict punishment, as well as its use as the verb to use a knout. It may not be commonly known, but it is a word and should be considered as valid in determining the four groups.

I do believe it is probable that more than four sets of groups of four can be formed that conform to the given puzzle's criteria. One such solution is as follows:
KN - KNAVE  KNEEL  KNOWN  KNOWS
BR - BRAID  BRAND  BRIDLE BROTHER
QU - QUANT  QUASH  QUEASY QUIRK
ST - STABLE STAIR  START  STOUT

A second solution could be as follows:
KN - KNEEL  KNOUT  KNOWN  KNOWS
BR - BRAID  BRIDLE BRASH  BROTHER
GR - GRAND  GRANT  GRAVE  GREASY
ST - STABLE STAIR  START  STIRK

For those unfamiliar, a stirk is yearling bullock or heifer (in British English).

A third solution could be as follows:
KN - KNAVE  KNEEL  KNOUT  KNOWS
BR - BRAID  BRIDLE BRASH  BROTHER
CH - CHAIR  CHANT  CHART  CHIRK
UN - UNABLE UNEASY UNIDLE UNOWN

To chirk is to make cheerful. The word unidle -- (to make or become) not idle -- can be found in "The Collaborative International Dictionary of English" and other unabridged dictionaries.

Edited on August 18, 2016, 12:50 am

Posted by Dej Mar on 2016-08-17 01:53:38