Artists sometimes struggle to make relatively straight lines freehand. A trick you might try is to completely lock the wrist and fingers, and instead, rotate your arm only at the elbow. You’ll quickly find that there’s a diagonal area that averages out to a mostly straight line with a very slight curve, just spin the drawing around until the placement of the line matches up with the movement of your arm. Once you get used to it, try bending the wrist in and out sightly while you draw that line, and if done properly, you can actually remove the very slight curve and straighten out that line. It won’t ever be a 100% perfect line, but a hand drawn line is fast, convenient, and I tend to prefer them over perfectly ruled lines because each and every one has a bit of character and originality to it.