In the examples mentioned above, 了 functions as an aspect marker.
The aspect marker 了 indicates that:
- an occurrence Pi of a process P (an action or an event) is located on the time line.
- Pi is a distinguished/distinguishable value of P (not just any occurrence of P).
By default, the time line where Pi is located is adjacent to the moment of speech (i.e. "hic et nunc"), but it can also be a segment in the past (for example: 昨天, 去年) or even a projected time in the future.
An occurrence is distinguished/distinguishable* if:
A) It is pre-defined (or presupposed). For example:.
- 我问了，他说不知道。 (I asked the question that was supposed to be asked.)
- 没有。该说的我都说了。(I said what I was supposed to say.)
B) It is delimited. For example:
In (3) and (4), as telic processes with a terminal point，去银行 and 咬死 are delimited by default.
In (5) and (6), 买 and 看 are delimited through quantification (几本小说, 两个小时).
Therefore, when we use the aspect marker 了, we don't just indicate that there is an occurrence of P, but that this Pi is a distinguished/distinguishable occurrence of consequence.
If we say: "昨天吃完晚饭后，我们去维多利亚港看烟花。", we simply mention that there is an occurrence of "去维多利亚港" together with an occurrence of "看烟花".
If we say: "昨天吃完晚饭后，我们去了维多利亚港看烟花。", to interpret "去了维多利亚港" as an occurrence of consequence, we have to put it in a context like "昨天吃完晚饭后，你到处找我们吗？我们去了维多利亚港"。
If we say: "昨天吃完晚饭后，我们去维多利亚港看了烟花。", to interpret "看了烟花" as an occurrence of consequence, we have to put it in a context like "看烟花是我们原来的计划，我们完成了。". If we don't have a presupposed objective, we can make the occurrence distinguishable through quantification: "昨天吃完晚饭后，我们去维多利亚港看了两个小时的烟花。".
*Beside (A) and (B), there is another case: when P is a compact process as in the case of “你胖了”, “孩子高了”. For lack of time and space, we are not going to delve into it. Please see my other article in the thread "What is 了 doing in 这个表快了+time？".