You create a new structure with a basement and floor with a base elevation of 8". Use detail rectangle to create base size. Convert to walls. Next, copy the walls to first floor. Now create floor cut out for half the floor. Next create a topography line about 20' from the house at 10" elevation. When you do a walk through, the grass is visible in the cut out area.
If the grass appears while you are moving, this is the correct visualization => during the movement stage, all the components are in preparation mode and when you release the mouse button, you can see the finished rendering. If the grass appears all the time, then the walls in the perimeter are not perfectly fussed.
Wall base boards extend to the end of the wall. This should not happen when the wall is intersecting another wall. The base board should stop at the intersection. It should also stop at door casings. It does not.
This is not accurate. If a wall intersects another wall, the baseboard stops. The same happens with a door casing. See "Baseboards" image attached. Important: in order to avoid mistakes, when you apply textures, trims, colors, etc. be aware of the different SmartWand modes.
Ceiling heights should also be set by elevation instead of the shortest wall.
TFP includes 2 types of floors, ceilings, roofs, decks, etc: The Automatic Components, with their own rules and the Custom components where you can determine the settings. Automatic ceilings follow the shortest wall rule (IMO, this is great because you always know that there won't be holes between the walls and ceilings). Manual ceilings can be partial, full, elevated or not as well as hidden.
Often, when you have a large opening, you may need to use a 1/8 Wall to cover the floor in an opening between floors. I know you can do the other ways such as using floor tool, but the colors do not blend. This method means you have to turn off ceiling and create your own.
Not sure what do you mean? If the walls are perfectly aligned
(this is very important), there is no gap as you can see in the image "Walls 2 levels" attached. If you are using a Floor Cutout, notice that the elevation and thickness of the cutout must be the same elevation and thickness of the floor / ceiling.
TFP, as any program, has a learning curve. The best way to learn the program, IMO, is practice + practice + practice => Read and follow the manual (this is important), see the videos => try to create elements not related with your project, so you will become familiar with the commands and how to develop the different components.
If you are interested, I have several training e-books, one of them is free. See links in my signature.
Hope this helps.