What's the difference between "intervene" and "interfere"?

Well, you can, of course, look up the definitions for intervene and interfere, but if you need more than that...

When someone intervenes, they are generally coming in between two parties who are in conflict in some manner. By intervening, they help attempt to resolve the conflict between the two parties. So, for instance, you could have two people arguing, and someone else intervenes to try and stop the argument. They are helping, and it has a positive connotation.

On the other hand, when someone interferes, they're just getting in the way. It doesn't necessarily have anything to do with two parties in conflict or even more than one party at all. Someone who interferes is getting in the way of someone else doing something. So, if I'm trying to get my work done, and you interfere, you're stopping me (or at least impeding me) from getting my work done. There's nothing positive about it. It's entirely negative.

Intervene has a positive connotation, while interfere has a more negative one. To interfere can mean to restrain, while intervene more leans towards to come between (lat. inter: between, venire: come), to engage. Interfere comes from lat. fere: to do.

One could say that interfere means to go between to do something, to restrain, disrupt, disturb with a focus on the doing part, the action that is done, while intervene means to come between to prevent, focusing on the motion part of coming between.