Please scan the last stanza of "Frost at Midnight" (detect the feet and the stress patterns within those feet). See below:Here is the stanza:...

I have scanned the paragraph as you have
asked.

I am mostly familiar with iambic pentameter, which is when a
poem has ten syllables per line, with the stress on every other
syllable.

Sonnets (fourteen-line poems) by Shakespeare, Petrarch,
Spenser, etc., were written in iambic pentameter.

In your poem, if I
have counted correctly, it appears to me that all the lines expect
one have ten syllables per line, with a stress on the second of the two
syllables.

For instance, refer to the first line of the
paragraph:

Therefore
all seasons shall be sweet to thee

The
stress lies on: -fore, sea-, shall, sweet, thee.  The stress does not follow words, but
syllables. There are times (as with the first two listed above) that the first syllable is paired
with part of the following word, or part of a word is joined with the word following
it.

So "Therefore" has two syllables, and "-fore" is stressed. The
next foot is "all sea-" and "sea-" is stressed. The second half of "season" (-son) is the first
syllable of the next foot: "-sons shall."

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There -fore / all sea- / -sons shall / be sweet / to
thee

This continues up until the line
below.

Smokes in the
sun-thaw ; whether the eave-drops
fall

This sentence does not follow the
pattern. Now, this is written some time between the end of the 18th Century into the middle of
the 19th Century. Coleridge was one of the two poets who are considered the forerunners of the
Romantic literary movement in England.

I mention this only because
poets (Shakespeare is a perfect example—and Coleridge as an English writer would have been
influenced to some extent) would sometimes write in iambic pentameter, but some lines would have
ten syllables, some would have eleven, and others might only have nine. The "irregular" numbered
"feet" did not dominate the poem, but they were there.

For instance,
in Sonnet 29, Shakespeare's third line is not written in iambic
pentameter:

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And trouble deaf heaven with my bootless
cries,

It is possible to
surmise that the line shown above (from "Frost at Midnight") simply does not fit the pattern
because Coleridge wanted to say something specifically that would not fit with the prescribed
number of syllables.

Sometimes poets (and song writers—who write
poetry to music) will add a word and rush two syllables together to "fit." Consider the word
"o'er." This is pronounced "or" (a one-syllable word), but actually means "over," a two-syllable
word. So perhaps Coleridge intended that when the poem was read, one of the two-syllable words
would be "rushed" or "pushed" together.

Considering the possibility
that Coleridge purposely chose a different number of syllables, I am unable to chart the line
which has eleven syllables. To make it fit into the ten syllable pattern (with a stress on the
second syllable), "whether" would need be said quickly—seeming the only place where such a "rush"
can be used and the line not only still make sense, but be able to hold onto the rhythm presented
in the other lines as well.

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Smokes in the sun-thaw ; whether the eave-drops
fall

spoken "Smokes in / the
sun / -thaw whether / the eave- / drops
fall"

The only other possibility I can
perceive might be that the foot "-thaw whether" might be an "anapest" (three
syllables, with stress on the last), considered a "trisyllable" (whereas iambs are
"disyllables").

I hope this is of some help.