Reservations and Exceptions

1a. While you should certainly "listen to the rhythm that your proposed scansion produces and to decide whether that rhythm could be what the poet would have wanted," remember that some poets commit blunders. Matthew Arnold, for instance, committed this atrocity in "To a Friend": "Who prop, thou ask'st, in these bad days my mind?" Just try saying it aloud, and the next line is even worse rhythmically although there is nothing in it to match the horror of "ask'st": "He much, the old man, who, clearest-souled of men. . . ."

2a. Although in theory "there can be no more than eleven syllables," poets do sometimes tolerate unmetrical lines, with extra syllables scattered here and there:  "I grant you that.  But my judgments's not so weak" and "I mean to get him out of his present straits" (Richard Wilbur, translation of Tartuffe).  As for the eleventh syllable being the only possible extra one, this is certainly true of Milton, but some Romantic and Victorian poets permit an interior extrametrical syllable after a pause: "Lost in a quagmire!  Many of you, yea most" (Tennyson, Idylls of the King).

3a. Although it is common sense not to "violate the stress pattern of a word," a poet may on rare occasions do just that.  It cannot be done casually, or readers would become hopelessly confused, but in a passage like the following, which deals with a Tarzan-type taming wild animals, Spenser trusts his readers to perceive an analogy between the beasts and the words, which yield very unwillingly to their masters.  When even the "Tigre cruell" is forced to submit to the yoke, Spenser's verse likewise forces the words "tiger" and "cruel" into an unnatural stress pattern so that "cruel" rhymes with "compel," but the task is not accomplished easily, for even in submission, "tiger" growls: "ti-GERRR":

[He could make] The Lyon stoup to him in lowly wise,
(A lesson hard) and make the Libbard sterne
Leaue roaring, when in rage he for reuenge did earne.

And for to make his powre approued more,
Wyld beasts in yron yokes he would compell;
The spotted Panther, and the tusked Bore,
The Pardale swift, and the Tigre cruell;
The Antelope, and Wolfe both fierce and fell;
And them constraine in equall teme to draw.

4a. The other side of this advice is that some poets may be clumsy at rhythm.  Even very sensitive poets can now and then nod off to sleep, as Keats seems to have done in "How many bards gild the lapses of time," whose two adjacent inversions (trochees: ´ ˘ | ´ ˘) make the verse break into a skip and then a stumble at the end.  Nevertheless, before charging a poet with a fault, it is always best to make sure that you have made no mistake yourself.