this is iambic (unstressed+stressed repeated). I'll use S for stressed and U for unstressed but please use the appropriate markings in your response. Remember that major words are more likely to be stressed, minor words (prepositions, articles, etc.). Also consider the natural stress pattern of a word.
When (U) fish (S) es (U) flew (S) and (U) for (S) ests (U) walked (S)
note that in the word "fishes" and "forests" the first syllable gets the stress (after all, you don't say it like fiSHES and foRESTS). "and" is unstressed because it's not a major word.
