The students found the grammatical rules incredibly simple to understand and use.
which word does the underlined phrase modify?

The word that the underlined phrase "grammatical rules" modify is rules.

Explanation:

Taking in consideration de phrase "incredibly simple", the direct noun that is modified by this adverb and adjective is rules, since they describe how the rules seem to be to the students, "The rules are incredibly simple to understand", "incredibly" modifies "easy" and then the two words together are connected to "rules."

"get off of your lazy bum and come running with me or i'm dumping this pitcher of water on u " u say with a smile. she thinks your joking so she ignores you about 10 seconds later shes screaming wile water is dripping everywhere you say "here comes the second one! " then you here her say okay, your such a j e r k ! ill be ready in 5 put the pitcher down " she say in an angry tone. and you smile in victory

good enough?