Hi,
<%= @user.nil? "not logged in" : @user.name%>
so if the user is not logged an appropraite text is echoed. I've tried print (which ends up doing strange things) and puts (which gives me a 500 error no less). How is this done?
your approach is good, but you got misleaded by the question mark.
nil? is a method of object, so the whole name of the method is nil?, with the question mark included.
if you want to use the operator ? the syntax is
condition ? result_if_true : result_if_false
the trick here is that in your case the condition would be @user.nil? and then the syntax would be
<%= @user.nil? ? "not logged in" : @user.name%>
notice the extra ? you didn't have before. So.. you were really close. Because of ruby accepting ? as a method name i had a hard time at the beginning using the ? operator coming from other languages. Finally I got used to use it always like
(condition) ? (if_true) : (if_false)
this way, by using the brackets, it makes easier to have the right syntax at the first try 
And, anyway, to complete the answer to your question... for really really really special cases where you cannot just use <%=%> syntax for some obscure reason (usually complex helpers with blocks), you could use the concat method. As documentation says, the <%=%> standard erb syntax is preferred.
Regards,
javier ramirez