First off, the function word needs to be before the rest of the code or else when you call word() it would be undefined and error.
The string.split() method splits a string on the word boundary that you pass to it. If nothing is passed, split() defaults to space. split() splits the string into a list:
foo = "Hello World"
bar = foo.split( ' ' )
print bar
[ "Hello", "World" ]
The word function just needs to iterate ( a for loop ) through sentence and
if sentence[ ] == sentence_1:
print "Yes"
return
elif:
pass
print "Nope"
