a big headache 令人头痛的事情（例：Dealing with complex grammar is a big headache for many students）

a fraction of 一部分（例：A fraction of the budget is allocated to English training）

above all 首先，尤其是（例：Above all, you need to master these key phrases）

absent from 不在，缺席（例：He was absent from the class due to illness）

back and forth 来回地，反复地（例：The students walked back and forth between the library and classroom）

by accident 偶然（例：I found this useful phrase by accident while reading）

by all means 尽一切办法（例：By all means, you should memorize these key points）

by no means 决不（例：This method is by no means the best choice）

cut down on 减少（例：We should cut down on unnecessary vocabulary memorization）

due to 由于（例：His poor score was due to lack of phrase accumulation）

in terms of 就...而言（例：In terms of exam application, these phrases are crucial）

on the other hand 另一方面（例：Phrases help in reading; on the other hand, they also improve writing）

wolf down 狼吞虎咽（例：He wolfed down his breakfast to catch the school bus）

rain cats and dogs 瓢泼大雨（例：We had to stay indoors because it was raining cats and dogs）