[uku25]

Which engine draft or production gives me a better quality? And why other material like stone or wood "grow" like grass, although grass is different component and group?
And last problem is that when I use HDRI sky, them sometimes ground and details are not gonna render and will appear as a black. What should I do?

[JDHill]

You should use the draft engine while you are working, and the production engine when you are rendering your final image. The reason is that production engine will feel slower for working, but will render to higher quality in a shorter period of time.

Grass is a special feature, which is separate from the idea of materials. Where a material is something you apply to geometry, the grass feature is actually a geometry generator -- it physically builds blades of grass inside the engine, according to its parameters. Regarding materials which are supposed to represent grass or stone, these may be built using the Displacement feature. This also generates geometry, but in a different way: according to the black & white values in the displacement texture, parts of the mesh to which the displacement is applied are moved some distance in 3D space.

On the HDRI question, I am not sure what you are describing, but it sounds like you may want to check the settings for the Reflection, Refraction, and Illumination channels. These can be set to use the same file and settings as the Background channel, or they can be set to use different ones. If this does not help, please send me a simple SketchUp file (jeremy at nextlimit dotcom) which shows the problem, so I can give you better advice on how to fix it.

[chedda]

I suspect they are blacked out because you need to boost the intensity of your hdri.