A Yoda-speaking challenge, this is
Retina, 162 137 145 134 131 129
Multiple punctuation is now handled correctly. Test case: You will not win this challenge...! -&gt; Not win this challenge..., you will.
Try it Online!
[.!]$

T`A-Z`a-z`^\w\w
G`^(I|we|you|s?he|it|they) \S+ \S
(\S+ \S+) (.*)
$2, $1.
T`a-z`A-Z`^.
^$
Too difficult, this sentence is.

Description:
If there is punctuation in the end, delete it.
[.!]$

^empty line

Transform the pronoun to lower case, except it is I.
T`A-Z`a-z`^\w\w

Filter out any line that does not match &lt;Pronoun&gt;&lt;space&gt;&lt;word&gt;&lt;space&gt;&lt;word&gt;...
G`^(I|we|you|s?he|it|they) \S+ \S

Split into pronoun + verb, remainder. Then rearrange.
(\S+ \S+) (.*)
$2, $1.

Transform the first character to upper case.
T`a-z`A-Z`^.

If the result is empty, it didn't pass the filter above. Print an error message.
^$
Too difficult, this sentence is.