The verbal suffix 了 indicates that:
- an occurrence of a process P is located on the timeline, and that
- this occurrence is a distinguished value (or at least a distinguishable value) of P (i.e. that is, it is not just any Pi).
In (1), with 过, it is the experience of eating durians that counts (that is, any Pi as long as it verifies the property of being P).
In (2), with 了, the quantification (by virtue of 两 and the classifier 个) renders the occurrence Pi individualised and hence distinguishable (i.e. it is not just a Pi like any other).
With a telic process (i.e. a process that is usually associated with a terminal point) such as 去，来，抵达，离开，割，剪，死...etc., the terminal point is by default the distinguished value:
- 她去过中国。(She has been to China)
- 她去了中国。(She has gone to China)
In (3), with 过, only the experience of "going to China" is relevant, not the terminal point of "going until being in China".
In (4), with 了, the terminal point of "going until being in China" is relevant.
If the process is associated with a target or objective, the target can serve to define Pi as a distinguished value:
In (5), "taking medicine" is something that you are supposed to do. In (6), "revising English and Math" is a task that was expected.