だんだん

The word だんだん is used to describe a state that changes slowly over time, and is usually translated as "gradually" or "little by little." It often occurs alongside the ていく and てくる forms because of this.

毎日日本語で話すようにしていると、だんだん上手に話せるようになりますよ。
If you make an effort to speak Japanese every day, you'll gradually get to where you can speak it well.

祖父は毎日英語を勉強して、だんだんペラペラ読めるようになったんだわ。
Grandpa studied English every day, and he gradually got to where he could fluently read it.

せっかく４年間も日本語を勉強したのに、大学を卒業してからあまり使わなかったら、だんだん忘れてしまった。
Even though I specially studied Japanese for four years, when I didn't use it very much after graduating from college, I forgot it little by little.

リーさんはジムに行き始めて、だんだんすごく強くなったんだな。
Lee-san started going to the gym, and little by little he got to be very strong.