I somehow managed to get Bs in two calculus courses and was pretty confident that in a pinch I was capable of doing work involving lagrange multipliers, Green's theorem, convergences of improper integrals, and such. My textbook was multivariable calculus by Larson and Edwards which I was sold forever ago. This semester I just enrolled in a Linear Algebra + Ordinary Differential Equations course so I figured I would need to go back and get a good reference for filling gaps in my knowledge and reviews on Amazon and this forum pointed to Widder as a cheap, highly worthwhile purchase. I'm having a much harder time with this than I thought I would. Do I need to go back to basics and pretty much relearn calculus? Did those courses I take simply fail to prepare me for serious work? Is there a certain approach I should take with this book? This seems so much more abstract than what I remember doing. I have the Tenenbaum book on ODEs which is just perfect for me and I'm enjoying so far and yet somehow Widder is overwhelming. I do recall seeing an old post somewhere mentioning a difference between doing Calculus and truly understanding Calculus at a more fundamental level.