How to make
isearch-backward) search only for words that start with the argument passed to them, instead of searching for the argument even if it's in the middle of a word?
word-search-mode is the usual way to constrain your searches to words rather than letters, as @rpluim shows.
An alternative, more general approach is to use regular expression searches,
C-M-s, and also the reverse
C-M-r. You can then use a regular expression to limit your search:
\bin will match
in when it occurs at the start of a word.
This is not as convenient as
word-search-mode, but it is more flexible. For instance, you could find words ending with