Why shouldn't the word partly be divided by a hyphen?
a.
do not divide a one-syllable word.
b.
do not divide proper nouns or proper adjectives. < awnser
c.
a two-letter word ending should not be carried over to the next line.
d.
do not separate a one-letter syllable from the rest of the word.

The word partly shouldn't be divided by a hyphen because C. a two-letter word ending should not be carried over to the next line. A is incorrect because partly has two syllables, and not one. B is incorrect because partly is an adverb, it is neither a proper noun (those are names), or an adjective. D is incorrect because the syllables in the word partly do not have one letter: part has four letters and ly has two letters.