As a general rule the more letters of RETAINS you have, the greater the chance of a bonus.
Below are all the bonus words you can make if you have six of those letters, RETAIN, and one other letter.
There are a lot of words, so to make learning easier, perhaps break them down into groups of 10-15 words or by distinct letter groups (i.e RETAIN+A, B, C, D and E).
A - nothing
B - ATEBRIN
C - CANTIER, CERATIN, CERTAIN, CREATIN, CRINATE, NACRITE, TACRINE
D - ANTIRED, DETRAIN, TRAINED
E - ARENITE, RETINAE, TRAINEE
F - FAINTER, FENITAR
G - GRANITE, GRATINE, INGRATE, TANGIER, TEARING
H - HAIRNET, INEARTH, THERIAN
I - INERTIA
J - JANTIER, NARTJIE
K - KERATIN
L - ENTRAIL, LATRINE, RATLINE, RELIANT, RETINAL, TRENAIL
M - MERANTI, MINARET, RAIMENT
N - ENTRAIN, TRANNIE
O - NOTAIRE, OTARINE
P - INAPTER, PAINTER, PERTAIN, REPAINT
Q - nothing
R - RETRAIN, TERRAIN, TRAINER
S - ANESTRI, ANTSIER, NASTIER, RATINES, RESIANT, RETAINS, RETINAS, RETSINA, STAINER, STARNIE, STEARIN
T - INTREAT, ITERANT, NATTIER, NITRATE, TARTINE, TERTIAN
U - RUINATE, TAURINE, URANITE, URINATE
V - nothing
W - TAWNIER, TINWARE
X - nothing
Y - nothing
Z - nothing
71 words in total. 6 letters RETAIN doesn't combine with to make a bonus (A, Q, V, X, Y and Z).