A true friend
（主语）
is
（be动词,相当于谓语）
someone you can talk to about any subject or problem.
表语从句
“you can talk to about any subject or problem”又做“someone”的定语从句.“someone”为先行词.
“someone you can talk to about any subject or problem” 的正常语序为：“You can talk about any subject or problem to someone”.因此“someone” 做“to”的介词宾语,“subject or problem”做“about”的介词宾语.
最好看看比较系统的语法书.其实语言是相通的,只要你把汉语的语法完全理解了,其它外语难题也就迎刃而解了.