I'm having a hard time when quoting comments -- I usually don't want to quote the entire comment when responding, just a line or two. But when I try to erase the rest of a quote, it'll usually erases everything. Is there a way to make this easier?

I usually left click my mouse and drag it over the text, this highlights it and then hit the delete key to delete the text. There's other ways like placing your courser either before or after the text and hitting the back space or delete key. You can always get out of a post that is messed up by hitting the back arrow on your browser before submitting the post, and then start over.

Yes, but when I delete the text, it seems to delete everything -- maybe it's because I can't see the selection?

 

I'm also on a laptop using a fingerpad, which makes it hard to be precise.

When you make a post, there are some toolbars with formatting controls. If you click the one at the top left (it looks like a light switch) it will disable the graphical representation of the quotations and just show the raw forum tags. I usually find it easier to cut out parts of text from quotes that way.