Trim not working
EDGEMODE = 1
TRIMMODE = 1
These are the usual solutions when trim is not working with extension of the lines. Other issues that I have encountered are lines not being on the same plane, i.e. they are on different elevations, or some lines are 3DPOLYLINEs.
If the lines being on different elevations is unintentional, just select all the lines, and then in properties, set elevation to 0 for all.
For turning 3DPOLYLINEs to 2DPOLYLINEs, easiest way is to download a lisp script. I found one that does just that. Search online for pline-3d-2d.lsp, I found it here:
Hatch not working
Hatch can have a lot issues, sometimes there are very small gaps in what seems to be a closed boundary.
The worse issue I have come across, which took a while to figure out is if you have multiple coincident points, which are adjacent in the vertex array of a polyline, then AutoCAD refuses to work.
Model Space linetype scale not matching in Paper Space
Matching Hatch scale in Model Space and Paper Space
Don’t copy paste from Model Space to Paper Space or vice versa. Change space (CHSPACE) through a viewport for automatic translation scale factor.
Getting rid of the ribbons and activating the classic menu and toolbar icons
If you have been using AutoCAD for a long time, and have developed speed using toolbars and menus, you will find that ribbons cannot match that efficiency. They may have been developed for touch screens, but a mouse is the most effective interface device for AutoCAD’s GUI. To AutoDesk’s credit, they are still keeping the full range of input methods available, and people can choose what they want (unlike Microsoft! Completely messed up MS Office). Though in many versions you have to set things up step by step rather than just asking AutoCAD to restore the classic interface. Though I have come across a utility on AutoDesk website that did it easily, not sure if it was developed by AutoDesk. There are plenty of online tutorials, so just search for them online.
AutoCAD free alternatives
AutoCAD being very expensive for personal use, it seems to be aimed at companies with sufficient funds. Even though AutoDesk is quite good with providing trial versions and educational versions, I haven’t come across a cheap(er) option for hobbyists and enthusiasts like they do for Fusion 360 now.
To fill that gap, sometimes you can come across clones of AutoCAD. At some point Draghtsight 2D was free from Dassault Systems, but they have started charging now.
These days, you can get NanoCad for free. It’s another AutoCAD clone, and it’s quite good. But it still has small bugs here and there. Maybe they will start charging too when they have ironed out these small issues.