By vs With - Any help appreciated!

[Sinjin]

Hello,

One of my students asked for clarification over when to use "with" and when to use "by". Specifically:

The bread was cut WITH a knife
The man was killed BY a hand grenade

If anyone can provide any grammar rules for these behaviours, that'd be great!

Thanks!

[Tdol]

There is a bit of a grey area here, but 'by' indicates the actual agent of the action and 'with' indicates the tool, etc. In the first, the person cut the bread more than the knife and in the second, the person threw the grenade, but the grenade was more active in the killing. If the person had thrown a stone and killed him, then you could say 'with'. However, different people may use these differently.