用 “would like to” 提出 “想要” 投诉。

I would like to speak to the manager.

I'd like to complain.

用 “I am unhappy about + 名词短语” 的结构表示 “我对…不满意”。使用副词 “very” 或 “really” 来加强语气，表示 “非常” 不满。

I'm unhappy about the food.

I'm really unhappy about the food.

I'm very unhappy about the quality of the product.

用 “It's unacceptable that + 从句” 的结构表示 “… 是不能接受的”。

It's unacceptable that you forgot my order.

It's unnacceptable that you expect me to pay for the damaged goods.

注意，不要 “lose your temper（发脾气，发火）”。

stop shouting – don't lose your temper.