vocabulary: 

where do I begin ( This phrase is usually used when you want to begin your complaining, in negative situation)

short-staffed 人手短缺的 

under-staffed 

we are a little bit short-staffed at the moment. 

You need an attitude adjustment. 

I specifically asked/stated.... (used when you start judging someone's service or manners which are different with what you expected or ordered. )

Could you state what you want done more specifically? 

swamped 忙的不可开交

the kitchen is swamped 

Sorry I did not get back to you. I have been really swamped at the work lately. 

get back to you 回复你

lately here means recently, which I think is a great work in oral speaking. like, what did you do lately.

pride ourselves on

we are pride ourselves on the hygiene in kitchen. 

on the house 免费

don't worry, it is on the house. 

complaint n. 

take on time at all 

take forever to do 

sanitary 卫生的

hygiene 更加偏向于指厨房类，食物的卫生 (adj. hygienic)

sanitary 倾向于指卫生间的卫生

meet expectation(差强人意的意思，也可以说让人满意) like, you just meet our exception, but we are think you could achieve more than that. 

to make better worse (present another statement in your complaint or to express your dissatisfaction )

20% off coupon for the next time 

send out the invitations to the party(分发邀请函)

but my... is overcooked/ too salty/ inedibly spicy 辣到不能吃 (that is kind of a way to start your complaint in a restaurant)

what you should do while you are complaining is to be specific about a resolution, if you have on in your mind to resolve the situation to your liking (definitely within reason!!), feel free to say it. the manager will do it if it is within the power. it is in their best interest that you leave satisfied. 

Useful sentences: 

May I ask what seems to be the problem? (very polite way to ask what is wrong here)

There is no excuse for the server's rude behavior. 

Is there anything else that is not meeting expectation? 

we pride ourselves on keeping a clean and hygienic kitchen, but of course these things happen from time to time. 

I am afraid that in the future we might have to take our business elsewhere. 光顾别家

should you decide to come back. (should here is a decent or formal way to substitute if. It is a great word to use when your are talking to your clients or customers, or other times when your need to be a bit more formal.)

I will leave the book on the table, should you change your mind to read it. 

Should you have any other questions, please do not hesitate to contact us.