1.【At】present,they are traveling in Europe.
at present ,目前 现在.目前,他们正在欧洲旅行
2.The train will arrive【不填】here in three minutes.
here 是副词,前面不用介词
这列火车将会在三分钟后抵达这里
3.Most【of】the young people will take part in the performance.
most 后面的名词前面如果有定冠词,则接of
4.【As】usual,he came very early.
as usual,和往常一样
5.What will you say【on】that occasion.
on that occasion 在那个时候；在那个场合下