1.advice, advise.
advice 是不可数名词，a piece of advice，advise 是动词。
This is my advice to you.
I was advised to purchase an economical car.

2.affect, effect.
affect 是动词; 意为influence. effect可以是动词也可以是名词。动词effect意为cause to happen, 名词effect意为the result。
Smoking affects health.
I was deeply affected by the news.
What is the effect of the movement?

3.farther, further.
Farther 指距离; further 表示时间，程度和数量等概念。
Chicago is farther north than Fort Worth.
Call this number if you want to get further information.

4. in regard to, as regards, 或 regarding
I am writing in regard to (or as regards, regarding) your letter of May 10.

5.in spite of, despite都是介词,后面不跟从句。
Despite the bad weather we enjoyed our holiday.
In spite of the rain, he didn’t bring an umbrella with him.

6.percentage, percent. percent跟在数字之后
A large percentage of my money is spent on food.
Almost fifty percent of the students attend the concert.

7.principal, principle.
Principal可作名词或形容词,意为chief official or main. Principle仅作名词,意为fundamental truth 。
In his research he followed basic scientific principles.
The principal side effect of decongestants is drowsiness.
The chairman pointed out his principal objections.

8.wait on意为serve, 不要与wait for混淆。
My friend was late; 1 had to wait for him for half an hour.
Good waiters are willing to wait on customers。