Uses of the motion verbs （行く、来る） with the te-form

Question

I'm studying with Tae Kim and it says that you can use a verb in te-form + 行く、来る to show that an action is oriented toward or from someplace.

In one example it says:

日本語をずっと前から勉強してきて、結局はやめた。
Studied Japanese from way back before and eventually quit.

And my question is why the 来る verb is conjugated to the te-form きて. I thought that only the verb that is before has to be in te-form. And what would be the meaning if I conjugate 来る in past:

日本語をずっと前から勉強してきた、結局はやめた。

Answer 1

The た-form would be correct if the sentence ended there.

日本語をずっと前から勉強してきた。結局はやめた。

The compound verb 勉強してくる is used in its て-form to connect these two sentences into one, in one of the most basic functions of the て-form.

日本語をずっと前から勉強してきて、結局はやめた。

The sentence sounds a bit unnatural to me, though. I would probably use an adversative conjunction.

日本語をずっと前から勉強してきたが、結局はやめた。