Understanding 了: difference between 她上个月去了北京 and 上个月我去台湾了? This grammar page tries to explain when to use 了 after a verb and when to use 了 after an object.

It claims when the time is specified, it's acceptable to use 了 after a verb and before the object as in 她上个月去了北京.

However, it provides another example where the time is specified yet 了 comes after the object (上个月我去台湾了).


When the time is specified, where does 了 go? After the verb or after the object?
How does the different 了 placement change the meaning/correctness of these two examples: (a) 上个月我去台湾了 and (b) 她上个月去了北京.
了 indicates 'completion of an action'

In "上个月我去了台湾" the action of "去" is completed, meaning you had gone there but it doesn't say you are still there or have already came back 

In "上个月我去台湾了",  '了' is a final particle for the emphasis (stress in tone) of the whole sentence 

You can see the different by compare the two structure with other context:


"下个月我去了台湾" is nonsensical, you can't already gone there when you say 'next month'
'了' in "下个月我去台湾了"  is a final particle for emphasis, and this sentence does make sense