Grammar JLPT Level N4 - Lesson 1

Structure of どんどん:

どんどん + （と） + Phrase

Details about どんどん:

どんどん (or ドンドン) is one of the countless words in Japanese that represent a particular sound or sensation. These words are known as onomatopoeia (オノマトペ in Japanese). どんどん represents the sound of banging (like on a drum), or the pounding of feet. In this way, it is regularly translated as 'rapidly', or 'quickly'.
Like だんだん, どんどん may be used before almost any phrase.

彼女(かのじょ)が出来(でき)てからどんどんお金(かね)が減(へ)っていく。
Since I got a girlfriend, my cash has been rapidly decreasing.
どんどん頼(たの)んでね！
Quickly order! (Literally means 'more and more' in a sentence like this, with the nuance of 'don't hold back, order in rapid succession')

While どんどん can take the particle と, it is not required. In fact, と is very often omitted from adverbs/onomatopoeic words in Japanese.

バスの運転手(うんてんしゅ)がどんどんと減(へ)っている。
The number of bus drivers is rapidly decreasing.

どんどん

Usage Notes for どんどん:

Initially, it can be easy to mix-up どんどん, with だんだん. だんだん is used to express slower changes, and may be thought of as similar to 'steadily', or 'step by step' in English.

毎日(まいにち)ピアノを練習(れんしゅう)しているから、だんだんピアノが上手(じょうず)になってきた。
Because I practice the piano everyday, I have steadily gotten better at playing. (Slow progression)
毎日(まいにち)ピアノを練習(れんしゅう)しているから、どんどんピアノが上手(じょうず)になってきた。
Because I practice the piano everyday, I have rapidly gotten better at playing. (Rapid progression)