The pyrrhic/spondee substitution

[milo]

The rhythm makes me think of Emily Dickinson.  Is her favorite first-line pattern the same thing as above, or is it something else? 
"I heard a fly buzz when I died," "I started early --- took my dog," "It was not Death, for I stood up," "There is no frigate like a book," "The mushroom is the elf of plants," "A narrow fellow in the grass" ...

This is actually the more popular trochaic substitution.  I intended to do a thread on that next but  . . . eh, we will see.

It also makes me think of Joyce Kilmer's "Trees" 
"I think that I shall never see
a poem as lovely as a tree."
etc. 

Really, meter is just getting a set rhythm in your head.  You can do this easily enough by reading some metric verse or even just saying to yourself daDUM daDUM daDUM daDUM for a few lines.

These lines are perfect iambic tetrameter (4 sets of daDUM daDUM)

I think that I shall never see

daDUM daDUM daDUM daDUM


he ellides poem in L2 so it is "POME"

It seems like your examples have more syllables than these. Is that a necessary element, that it is done in a sonnet line? Or does it count in any form as long as the last four syllables do the dadaDUMDUM bit?

Rhythm is not my forte---trying to make sure I understand what differentiates one thing from another.  Thank you, this is a fun topic.

so, the most common meter for a sonnet by a long stretch is iambic pentameter which just means 5 iambs in a row.  You can use substitutions in tetrameter but it does become more noticeable because every substitution takes up 2 of your four feet .

try saying to yourself 

daDUM daDUM daDUM dada DUMDUM

a few times and see if you can come up with a nonsense line that follows that rhythm