What's the role of 曾经 in 他曾经在北京学过两年汉语?
曾经 is more equivalent to &quot;once&quot; in English as in the sentence

Once we were the best friends.

It is used to:


express the fact the sentence is talking about happened in the past.

if the predicate verb is a durative verb*, express that fact can't be experienced(经=experience) by the speaker(doesn't last) any more. 
, the speaker implies that the status is different now. In this case, it means &quot;used to&quot; as in the sentence &quot;I used to be a kind man(implies now I am not).&quot; For your example, the speaker implies that he is not studying Chinese in Bejing now.



*By &quot;durative verb&quot;, I mean the action expreesed by that verb can last for a long time. For example, &quot;study, like, look, be, work,feed, etc.&quot;. These verbs can be used in the progress tense in English(-ing form). Some other verbs can not, such as &quot;hear of, see, kick, etc.&quot; You can't say &quot; I am hearing of a terrible story&quot;, while you can say&quot; I am listening to a terrible story&quot;.
Can someone tell me the right grammar term if I am wrong?