So I'm trying to design a diagonal line pattern using the tool and grids to get a nice, even pattern, but even though I've enabled the grid and checked "Snap to grid" in every dialogue I can find (document properties and inkscape preferences), it still doesn't snap. I've searched through the help, this forum, and google, and I still can't get it to snap. Is there something I missed?

Welcome to InkscapeForum!

Hhm, in Document Properties, on the Snap tab, try ticking Always Snap for Snap to Grids, if you haven't already. And you didn't mention the Snap control bar. It may be located in different places in the Inkscape window, depending on what you have set in the View menu > Default, Wide or Custom (at bottom of menu). It may also be hidden, View menu > Show/Hide. Once you locate it, you need to click the very first button to enable snapping. Then you would click various other buttons depending what you want to snap and snap to. To draw a stripe pattern with the Pen tool, you'll need to click Snap nodes or handles and Snap to paths, as well as the grid button.

If you've got that all set, and still can't snap.....hhhm, well sometimes if you're zoomed out a lot, it's hard to snap, so you could try working at 100% or more zoom. If you still can't snap after that, your copy of Inkscape may be corrupt, which would require a simple reinstallation (well, simple if you saved the setup file, otherwise you'd have to download again).

But hopefully it's just that you don't have the Snap control bar enabled