Hi Supagoat, here are some screenshots showing the specific steps I used for your file.

Often before removing some pieces you will need to extract some curves from them, to use for cutting some of the rebuilt pieces later on. It's also often not bad to extend lines a ways so you know you have a good thing to cut any extended surface that you might build as well. So in this case before deleting the bad parts I drew 2 lines like this, snapping on to the midpoint of those edges there and using the "Both sides" option to get an extended line:



Then I moved those off to the side a bit to get them out of the way, then selected a bunch of faces (select faces by doing a "drill in" selection where you go and click on an object a second time, the first click selects the whole object and any second click "drills in" to select edge or face sub-objects. To target faces use your mouse in an area near the middle of a face that's not nearby any edge, if you're close to an edge the edge gets targeted), and deleted them. That left it looking like this:





Now to start with the reconstruction, select that edge shown above and run Construct > Extrude, and extrude it up, snapping on to the endpoint of the vertical edge there so you have the right height.

Now you can use Edit > Trim to remove pieces of this surface. Select the extrusion and run Edit > Trim. At the prompt to select cutting objects, select the 3 lines shown here, trim will project them onto the surface to cut it up (just let Trim do the projection itself, you don't need to do any separate manual projection step separate from that):




The next stage of Trim lets you pick which cut up pieces to discard (or you can pick which to keep by switching the option for it), pick this central piece as the piece to discard:




Then finish the Trim command and it will leave a result like this:





Repeat this same step on the other side as well, then draw in 3 connecting lines like this:




Now you have some planar regions to fill in, select this set of edges and that one connect line like this:



And use Construct > Planar to build a planar surface there, that will be the rebuild top piece of the bottom slab. Now you have 3 more planar regions to fill in the same way, like select these 3 edges and the one connecting line and run Construct > Planar on that too:




Now with all the surfaces reconstructed, select them all and run Edit > Join to glue all those together back into a solid. When you select the object, the object type indicator in the upper-right corner should say "Solid". If it says just "Joined srf" instead it means there still an unfinished area in it, use the "select naked edges" script (http://moi3d.com/forum/index.php?webtag=MOI&msg=6051.2) in that case to see where that area is.


So anyway, that's how I used the lower level tools to repair your particular object here, hope that makes sense!

It tends to be quicker and more convenient to use the "higher level" tools more up front, which means working more with solids and doing boolean operations so when you cut something it will leave the imprint of the whole cutting volume onto the other object. The low level surface editing tools like Trim can do the same thing but involve more manual steps but they can be helpful when you need to repair some portion of an existing object like this case. Trim lets you work on just one surface at a time and then when you're done use Join to glue them together.

- Michael