5 letter words:
first8,
frist8,
frits8,
rifts8,
4 letter words:
firs7,
fist7,
fits7,
fris7,
frit7,
rifs7,
rift7,
sift7,
rits4,
stir4,
3 letter words:
fir6,
fit6,
ifs6,
rif6,
sif6,
its3,
rit3,
sir3,
sit3,
sri3,
tis3,
2 letter words:
if5,
is2,
it2,
si2,
st2,
ti2,