1.He (started )earning a living by himself at the age of 14.
根据AT THE AGE OF 14 可知是一般过去时.
2.You mustn't let your (social 社会的,形容词后接名词‘LIFE) life get in the way of your studies.
3.It's inpossible (to finish) the work in such a short time.
注:it's +adj.+to do sth 固定用法
加油!不明白再问!
