There are differing opinions about Sprung Rhythm: there's much dispute, even among experts. Asking a poetry professor about Sprung Rhythm is like asking a racing driver about "understeer": you end up no wiser - because it depends so much more on practice than on theory.
There's a problem with traditional metre - such as iambics in which every other syllable is stressed - in that it is restricting to write lines with exactly the same number of syllables which have an alternating pattern of stresses. Shakespeare, on the other hand, did manage to do it fairly well. The result doesn't sound much like normal speech but it does sound like poetry.
Sprung Rhythm follows different rules which are more like normal speech. In general the rule is: unstressed syllables don't matter provided that there are a fixed number of stressed syllables in each line. The result looks less like poetry on paper, but sounds more like poetry when read aloud. It's possible to say a lot more about the theory, but if you think about the theory of cycling while riding a bicycle the chances are you'll fall off.
Hopkins sometimes indicated where he wanted the stresses to fall by occasionally putting a check mark over the vowels. Some people think that the first syllable of each line has to be stressed - but it ain't necessarily so.
The odd extra unstressed syllable hardly breaks the flow of the conventional line, anyway. There are plenty of Shakespeare's iambic pentameters which have an extra syllable. Words such a "the", "and", "in", "of" contain vowels but they can blur into consonants when read aloud, so make no real difference to the way the line scans.
Here's Gerard himself explaining Sprung Rhythm - you'll find it makes everything as clear as mud:
In this poem, Spring and Fall, Gerard indicates four stresses per line. So there's some liberty in choosing which four syllables to stress in the lines he didn't indicate. I added more stress marks to show which syllables I stressed when reading it.
If you want to type vowels with stresses, then put your finger on the Alt Key and type four digits on the numeric keypad - make sure NumLock is on.
á = Alt 0225
é = Alt 0233
í = Alt 0237
ó = Alt 0243
ú = Alt 0250
ý = Alt 0253
Á = Alt 0193
É = Alt 0201
Í = Alt 0205
Ó = Alt 0211
Ú = Alt 0218
Ý = Alt 0221
Here's the rest of these fancy characters you can make using the Alt Key - but don't expect every font to work according to these rules.
The photograph was taken in 1866, by Thomas C. Bayfield, and it is in the National Portrait Gallery
The autumnal picture came from this site
Margaret, are you grieving
Over Goldengrove unleaving?
Leaves, like the things of man, you
With your fresh thoughts care for, can you?
Ah! as the heart grows older
It will come to such sights colder
By and by, nor spare a sigh
Though worlds of wanwood leafmeal lie;
And yet you will weep know why.
Now no matter, child, the name:
Sorrow's springs are the same.
Nor mouth had, no nor mind, expressed
What heart heard of, ghost guessed:
It is the blight man was born for,
It is Margaret you mourn for.