有一天晚上，david偶然听到他父亲在弹钢琴，于是事情有点“不对了”。david说他想成为一名音乐家，而不是一名律师。(things to do)
one night david overheard his father playing the piano and then something seemed to be wrong. david said that he wanted to become a musician not a lawyer.
他那个关于计算机的理论在计算机投入使用前就已经为很多学者所知了。(come into use)
his computer theory has been known by many scholars before the computer was come into use.
一直到susan开口说话时我才知道她是外国人。(强调句型)
it is til susan spoke that i knew she was a foreigner.
那位长者考虑在给那个年轻人一次机会，既然他已经承诺不再犯同样的错误。(have...in mind)
since the young man has had his promise that he would never make the same mistake in mind the older considered to give him another chance.
这部电影不仅让人看了感到开心，也给我们上了一节道德课。(not only...but also)
not only did this film make people feel happy but also gave us a moral lesson
一看完这部小说，robin就赶到附近的书店买了一部影碟，该片改编自这部小说。(upon doing sth.)
upon finishing reading this novel robin hurried to a bookshop nearby to buy a dvd which was adapted from this novel.
张丽丽是黑龙江的一名高中老师。她舍身救下两名学生，这为她赢得了“最美丽的教师”称号。(win sb. sth.)
zhang lili was a high school teacher in amur river. she sacrificed herself to save two students which won her the title of the most beautiful teacher
所有乘客都必须出示车票，这已是通行的惯例。(be required to do sth.)
all the passengers are required to show their tickets which has been a common practice
jenny想到自己可能再也见不到nick了，于是眼里充满了泪水。(同位语从句)
the thought that jenny would never see nick again made her eyes full of tears
男孩醒来时，他父母已经去了工厂，临走时给他留了一张字条。(by the time)
by the time the boy woke up his parent has already gone to factory left a note to him
这次改革涉及技术和管理两方面，可能会使公司的利润翻倍。(in a position to)
this reform involves technology and management which is in a position to double our company's turnover.
周末你要和朋友出海听起来很有趣。(sound like + n.)
you will go to sea with friends at weekends. it sounds like a interesting idea
对学外语的学生别太过严厉，应该考虑到他们的母语和目标语言差别很大。(ought to)
we should not be overstrict with the students learning foreign language and we ought to consider that there are great differences between their mother language and target language
网上购物有别于传统购物，因为我们的顾客无需走出家门就可以在家挑选到她们喜欢的任何东西。(in that)
online shopping is different from the traditional shopping in that the customers can choose anything they like at home without going outside.

buyer will be responsible for any import duties imposed by the custom department of the destination country. however import duties are very rare for our products.

帮你修改了一下。因为你说的不过这是非常罕见的，就是在中文句子里也有“买家物品进口海关税”非常罕见的双重意思。翻译成英语后如果他们误会以为买家一般不负担进口海关税就麻烦了。

how do you explain the decline of the company's profits for three consecutive months
公司的利润连续三个月下滑你怎么解释