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).

RETAIN +

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).