When I enter a group, all read messages are gone. How to view them again?
If you enter the group by saying ‘RET’ in group buffer with point over the group, only unread and ticked messages are loaded. Say ‘C-u RET’ instead to load all available messages. If you want only the 300 newest say ‘C-u 300 RET’
Loading only unread messages can be annoying if you have threaded view enabled, say
(setq gnus-fetch-old-headers 'some)
in ~/.gnus.el to load enough old articles to prevent teared threads, replace 'some with t to load all articles (Warning: Both settings enlarge the amount of data which is fetched when you enter a group and slow down the process of entering a group).
If you already use Gnus 5.10, you can say ‘/o N’ In summary buffer to load the last N messages, this feature is not available in 5.8.8
If you don't want all old messages, but the parent of the message you're just reading, you can say ‘^’, if you want to retrieve the whole thread the message you're just reading belongs to, ‘A T’ is your friend.