MyBB Community Forums

i untick Can view board under user groups but it deny them to all forums.

how can i allow guest to access certain forums?

Go to: ACP > Forums and Posts > Select Forum > Edit [Options] > Permissions > Guests > Edit Permissions > and tick the following two options;
- Can view forum ?
- Can view threads within forum?

The 'can view board' permission disables guests from viewing everything on the forum, except the login page. (which is what they will see on every page until they log in) To allow them to view certain forums, enable the 'can view board' setting, and do what Yaldaram said.