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, INTERS, 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 INTERS+A, B, C, D and E).
A - ANESTRI, ANTSIER, NASTIER, RATINES, RESIANT, RETAINS, RETINAS, RETSINA, STAINER, STARNIE, STEARIN
B - nothing
C - CISTERN, CRETINS
D - SNIRTED, TINDERS
E - ENTIRES, ENTRIES, NERITES, RETINES, TRENISE, TRIENES
F - SNIFTER
G - RESTING, STINGER
H - HINTERS, NITHERS
I - nothing
J - nothing
K - REKINTS, SKINTER, STINKER, TINKERS
L - LINTERS, SLINTER, SNIRTLE
M - ENTRISM, MINSTER, MINTERS, REMINTS
N - INTERNS, TINNERS
O - NORITES, OESTRIN, ORIENTS, STONIER, TERSION, TRIONES
P - NIPTERS, PTERINS
Q - nothing
R - nothing
S - ESTRINS, INSERTS, SINTERS, STRINES
T - ENTRIST, RETINTS, STINTER, TINTERS
U - NUTSIER, TRIUNES, UNITERS
V - INVERTS, STRIVEN
W - TWINERS, WINTERS
X - nothing
Y - SINTERY
Z - nothing
63 words in total. 7 letters INTERS doesn't combine with to make a bonus (B, I, J, Q, R, X, Z).