since
prep.
自…以来； 自…以后
I have been there many times since the war.
自那次战争以来，我曾去该地多次。
"Since then, he has developed another bad habit."
"自那以后,它养成了另一种环习惯。"
"Since then, they have been trying to find out how the fire began."
"自那时以来,他们一直在设法查明起火的原因。"
- 词性变化
conj.
自…以后； 自…以来
What have you been doing since I last saw you?
自我上次和你见面以后，你在做什么？
既然；因 为
Since you ask, I will tell you.
你既然问，我就告诉你。
"Since many of the customers work during the day, Billy has to collect the money at night."
"因为许多顾客白天上班,所以毕利只好晚上去收钱。"
adv.
其后，后来
At first he refused but since has accepted.
他最初拒绝，但后来接受了。
We have been friends ever since.
自此以后我们一直是朋友。
"The fish, which has since been sent to a museum where it is being examined by a scientist, is called an oarfish."
"这条后来被送到一个博物馆,在那里接受一位科学家的观察、研究的鱼,名叫皇带鱼。"
以前
How long since is it?
那是在多久以前？
现代英汉综合大辞典
柯林斯
百科