Although N810 got hardware keyboard, I still want to use on screen one sometimes, such as type simple urls or search strings etc. This could be done easily by activate it in control pannel. When I done need it, again switch it off in control panel was easy and fast.
Now if I instll CJk, it wil completely disbale the original soft keyboard. If I install the matchbox keyboard come with CJK, then the soft keyoard will always show up have no way to disable it.
I'd like to know if there is a way to enable the classic one without to install matchbox, or if there is way to simply turn on/off the matchbox keyboard.
Thanks.

I use cjk on N800. Unfortunately, to my knowledge, there is no solution to both your requests. Back in OS2007, you can switch between built-in and matchbox keyboard. This feature is removed in OS2008. Too bad, I would also like to be able to switch between the two.

Alternative solution is to not to install matchbox, but install yet another on screen keyboard such as xvkbd. The trade-off, this is not meant for non-hildon applications. Thus, you have to run it manually. It won't automatically pop up in text entry. Search for it in this forum. Someone has ported it to the tablet.

For the keyboard side, one fairly smooth-working third-party option is to use the matchbox-keyboard from Qole's Easy Debian Chroot, with a simple start/kill toggle script in osso-statusbar-cpu or similar. I use that occasionally; it's especially nice for portrait mode.