First of all, welcome to the gentoo community. It's not an easy road, but it is worth it. I would advise you make an account on gentoo.org. They have a discussion board particularly for gentoo installs and all the users are more familiar with gentoo.
Nothing against linuxquestions.org. I use both sites.
second: I think you may be a little mixed up with your commands. I'm not going to lie taking on a gentoo install when you aren't all that familiar with the shell is a very steep learning curve.
I tried to kde4, the refused commands were "# emerge --pretend kde | less, # emerge kdebase kdenetwork kdeadmin,
$ startx, env-update " give the message that the BASH does not understand the command or invalid.
You aren't by any chance copying the hash mark in to the command line are you? The word "emerge" should be the first part of those commands. The hash mark just indicated that the command needs to be run as root.
after you have emerged kde, startx is the command to start the gui. This command should not be run as root, I haven't looked at the install guide in a while but I imagine by now you have already made your own user account. you need to start the gui from there.
The env-update command updates all the environment variables. This should also generally be run as a standard user since there is rarely a need up update the root accounts environment variables.
Also I have face some difficulty when I putted which start with /etc... or /usr..., what I think it is a folder path where the result message is tell me that this is directory I can say may be the lack of the knowledge of commands and gentoo file system (help me about this please).
I'm not really sure what you are talking about. Could you reply with the complete command line you had a problem with?