Machine malfunctions

Well Joe and I have decided to hire a baby sitter at least one night a week to allow us to do some outside yard work before the weather turns very cold. We have many projects that need done but often with 2 boys only one of us is working or none of us is working. So last night our babysiter came over for the second time. Joe ended up having to work, but I figured that is alright I can weedeat the fence (630 ft of fence on both sides is a lot). So I tried to start the weedeater, but ended up flooding it. No problem I will take the tractor and get a load of dirt to spread where we have been removing the rocks for the grass. So I go down the driveway and decide our weeds along the driveway are too tall and I should do a swipe with the mower, so I try to engage the mower and boom, the belt pops off. No problem this has happened before but I am not sure how to fix it so I go and get my dirt. I come back and park the tractor and decide to do another try with the weedeater. I end up parking the tractor o our slight hill in the backyard. I thought I had engaged the parking brake, but as I start to walk away, the tractor starts slowly rolling down the hill towards the fence. I run to the tractor, as the dogs are barking at it, and try to grab the handle to stop it (no luck) so I jump up on it and slam on the brake and stop about 7 ft from the fence (disaster avoided) so i go and get the weedeater started, yipee, and start to weedeat, I get half way around when it runs out of string and I tried for 15-20 mins to get the string thing off to put more string on, but i was unsuccessful. So I gave up and decided to go back for more dirt, as I got down to the pile Joe was pulling in and I gave up for the night. So it was an unsuccessful night for the yard work, but we feel it will get better!!!!