Gerunds vs Infinitives

Hi! I was doing some practice problems and stumbled upon one which I couldn’t really tell if it should be changed to a gerund, but I left it as an infinitive and got it wrong…
Here it is: … the new system changed the reasons to fight and the methods of combat.

Why does it have to be in the gerund form not in the infinitive ??

Don’t underestimate the power of “Search” (on top of the forum’s page); oftentimes the answer can be found in older threads.

http://talk.collegeconfidential.com/discussion/comment/19156820

Oh wow! Thanks so much that actually helps! But yeah lol ik what the “Search” button is, but for the most part I dont get what I want from it.