What is the うち in 使っていくうちに?


The following excerpt is taken from a conversation with my language partner:

> 「てform の使{つか}い方{かた}」* を読{よ}みました。このように書{か}いてあると、難{むずか}しく感{かん}じますね。  
>  私{わたし}たちが「てform」を使うとき、「この動詞{どうし}は意志{いし}を表{あらわ}すから」などとは考{かんが}えません。  
>
> 子{こ}どもの頃{ころ}から使っていくうちに、身{み}についてきたものだからです。でも、日本語学習者{にほんごがくしゅうしゃ}にとっては、分類{ぶんるい}してある方が、わかりやすいのでしょう。

The sentence in question:

> 子どもの頃から使って **いくうちに** 、身についてきたものだからです。

My attempt at translation:

> "From when one is a child, ..."

I must admit that I really can't make any sense of this sentence. I guess that
she's trying to tell me that they are using it from childhood on and that "it
just comes by itself" (the proper use of て-form), but I don't know. I can't
even tell the morphemes apart; is it "いくうちに" or "いく + うちに"?

And what about 身について? Is it 身 + について?

Maybe

> 身についてきたものだからです => "regarding oneself, because it is a thing that came."  
>  or "because it is a thing which came to oneself."

where I regard について as に+ついて, with ついて being て-form of 着く. I always feel a bit
uncertain about my understanding of this.

As I use it, it often resembles the use of は as a topic marker, translated to
"Concerning X". But from a grammatical perspective I think that the two are
quite different.

*(see here for the article she is referring to <https://www.learn-japanese-adventure.com/te-form-cause-reason.html> ) 




From [Jisho.org](http://jisho.org/search/%E5%86%85). Second definition of うち
reads:

>   2. while​
>

>
> Usually written using kana alone, See also ないうちに, also 裡
>
> あの[少年]{しょうねん}は[店]{みせ}から店を[歩]{ある}いている **うちに** [盗]{ぬす}み[癖]{ぐせ}がでてしまった。  
>  The young boy got sticky fingers when he walked into stores.

First definition of 身につく is:

>   1. to master (e.g. a skill); to become accustomed to (e.g. a lifestyle);
> to acquire (e.g. a habit)​.
>

This also uses the 〜ていく grammatical form.

My rough translation for the passage in question would be (feel free to
correct me):

> The て form is something we master after using it again and again from
> childhood.

The other things to be mindful of are the 〜てくる and ~ものだから grammatical points.




From [Jisho.org](http://jisho.org/search/%E5%86%85). Second definition of うち
reads:

>   2. while​
>

>
> Usually written using kana alone, See also ないうちに, also 裡
>
> あの[少年]{しょうねん}は[店]{みせ}から店を[歩]{ある}いている **うちに** [盗]{ぬす}み[癖]{ぐせ}がでてしまった。  
>  The young boy got sticky fingers when he walked into stores.

First definition of 身につく is:

>   1. to master (e.g. a skill); to become accustomed to (e.g. a lifestyle);
> to acquire (e.g. a habit)​.
>

This also uses the 〜ていく grammatical form.

My rough translation for the passage in question would be (feel free to
correct me):

> The て form is something we master after using it again and again from
> childhood.

The other things to be mindful of are the 〜てくる and ~ものだから grammatical points.

