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


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。




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。

