And then all of a sudden he could.
obsolete sentence page
I need to diagram this sentence and I am not sure how?
And than all of a sudden he could.
Your difficulties arise in part from the fact that
"And then suddenly he could."
is not a sentence. (I do not know what they are teaching today but I believe it is wrong to begin a sentence with a conjunction.)
Remove the "And," and tack on a verb (and toss in a few commas):
"Then, suddenly, he could fly."
Now you have a sentence. I am going to hazard two guesses: that "he" is the subject and that "could fly" is the predicate. I am also going to guess that "Then, suddenly," acts as an adverb phrase modifying "could fly."
Draw a horizontal line. Draw a vertical line bisicting it. Write "he" on the horizontal line to the left of the vertical line. Write "could fly" on the horizontal line to the right of the vertical line. Draw a slanted line going up and to the right from "could fly" and write "Then, suddenly" on it.
Maybe "could" should be on a slanted line pointed at "fly," but I cannot remember. Sorry...
I hope this helps.