Use of 了: 父亲为儿子买橘子，尽管十分辛苦，心里也是高兴的，越是尽到了父亲的责任，心里越踏实。

[anonymoose]

The 了 in this case does not reflect whether or not the fatherly duties have been fulfilled, but rather that the more he has fulfilled them or will have fulfilled them, the more assured he feels or would feel.

 

I'm tending to agree with you.

 

From Kenny's example above, that would also seem to be the case.

[Michaelyus]

The 越 ... 越 ... construction has an "unboundedness" effect, which means it cannot have a limit to the "correlation", which also excludes verbal phrases that are presented as "accomplishment". Here, that's exactly what the author is going for: the accomplishment. The way round this restriction is to present it as a state of 尽到责任, requiring the use of 是, and leading to what jiangping in #17 and Altair in #18. See the footnote in this 2008 paper.

 

So why the aspectual 了? It marks part of what Yip & Rimmington's grammar calls a narrative sentence, and introduces both that element of accomplishment and the definiteness of this particular "event". Without it, like in Wen87's post #8, the structure leads to what is called an expository sentence, suited to explaining things and for presenting things in a very general manner, as if to say the correlation between 尽到父亲的责任 and 心里踏实 could apply for all situations to this 父亲. By adding the 了, the connection between the correlation and the event is emphasised. However, this distinction that aspectual 了 brings is not something specific to its interaction with 越...越...: it applies wherever an aspectual 了 is possible in general.

 

It's quite an instructive sentence, I have to say, and it's great fun to analyse. 

 

Additionally, Kenny's sentence in #20 shows to what extent 了 can be used to give definiteness as well as the implication of past time and of completed action to something. It's also a lot more intuitive than the first sentence, the use of the definite reference in the form of 這樣 makes 了 a lot more natural than not having it there. 多谢！