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

SARNIE + 

A - nothing

B - nothing

C - ARCSINE, ARSENIC, CARNIES, CERASIN

D - RANDIES, SANDIER, SARDINE

E - nothing

F - INFARES, SERAFIN

G - ANGRIES, EARINGS, ERASING, GAINERS, GRAINES, REAGINS, REGAINS, REGINAS, SEARING, SERINGA

H - ARSHINE, HERNIAS, NEARISH

I - SENARII

J - INJERAS

K - SNAKIER

L - ALINERS. NAILERS, RENAILS

M - MARINES, REMAINS, SEMINAR, SIRNAME

N - INSANER, INSNARE

O - ERASION

P - PANIERS, RAPINES

Q - nothing

R - SIERRAN, SNARIER

S - ARSINES, SARNIES

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

U - nothing

V - AVENIRS, RAVINES

W - nothing

X - nothing

Y - nothing

Z - nothing

54 words in total. 9 letters SARNIE doesn't combine with to make a bonus (A, B, E, Q, U, W, X, Y, Z)