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).

INTERS + 

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 - REKNITS, 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).