what is the monosyllable rule?

Aha! According to the monosyllable rule, a stressed syllable can occur in a weak position only in two situations 1)if it is a monosyllabic word 2)if it belongs to a polysyllabic word, and the stress of this word occurs at the beginning of the line or after a major syntactic break. On the other hand, an unstressed syllable is always ok in a strong position.
Shakespear wrote in iambic pentameter. The stress pattern is: WSWSWSWSWS. In class we are writing out stressed syllables with capitol letters. So the first couple of lines of Hamlet's famous speech would read:
to BE or NOT to BE THAT is the QUES-tion
WHE ther tis NO-bler in the MIND to SU-ffer...
Those spots where there is a stressed syllable in a weak position are at the beginning of a line, and after a syntactic break.
How about the following sonnet- does Shakespear follow the monolsyllable rule? (that is: do the stresses of the polysyllable words fall on the even (strong) syllables?)
Let not to the marraige of true minds
admit impediments. Love is not love
which alters when it alteration finds,
or bends with the remover to remove:
O, no! it is an ever-fixed mark,
that looks on tempests and is never shaken.