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, TRAINS, 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 TRAINS+A, B, C, D and E).

TRAINS + 

A - ANTIARS, ARTISAN, TSARINA

B - BRISANT

C - NARCIST

D - INDARTS

E - ANESTRI, ANTSIER, NASTIER, RATINES, RESIANT, RETAINS, RETINAS, RETSINA, STAINER, STARNIE, STEARIN

F - nothing

G - GASTRIN, GRATINS, RATINGS, STARING, TARINGS

H - TARNISH

I - nothing

J - nothing

K - KIRTANS, RANKIST

L - RATLINS

M - MARTINS

N - nothing

O - AROINTS, RATIONS

P - SPIRANT, SPRAINT

Q - QINTARS

R - nothing

S - INSTARS, SANTIRS, STRAINS

T - STRAINT, TRANSIT

U - NUTRIAS

V - nothing

W - nothing

X - nothing

Y - nothing

Z - nothing

38 words in total. 10 letters TRAINS doesn't combine with to make a bonus (F, I, J, N, R, V, W, X, Y, Z)