童心未泯的英文：
to retain a childish heart; still to keep traces of childishness or childlike innocence

retain是什么意思：
v. 保持；保存；记住；雇用

orthodontic retainer
正畸保持器

I had one retainer.
我曾有一个牙齿矫形器。

Retained earnings is an owners' equity account that represents accumulated earnings retained in the business.
留存收益是所有者的权益帐，它表示留在企业的收益积累。

childish是什么意思：
adj. 孩子的；孩子气的；稚嫩的，幼稚的；天真的

Crying for things you can't have is childish.
想要的东西得不到就哭是孩子气的表现。

A stupid,disgraceful,or childish act or performance.
傻事，儿戏，轻率之举愚蠢的，丢脸的或孩子气的行为或表现

The vocal part of this song is sung in a childish voice.
这首歌曲的歌唱部分是用童音进行演唱的。

heart是什么意思：
n. 心，感情，热忱，内心，中心，红桃

They exchanged their views heart to heart.
他们推心置腹地交换了意见。

Distance is naught while heart with heart can meet.
心心能相印，距离等于零。

get/learn by heart
记住,背诵