成语【chéngyǔ】

A chengyu is a fixed four-character idiom, and in a sentence it works as one word. Which slot it fits depends on whether it behaves like an adjective or like a verb.

As the predicate

Most chengyu stand straight after the subject, with no 是.

Before a noun, with 的

Before a verb, with 地

Chengyu about manner or mood take 地 and describe how something is done.

After 得

A chengyu can also be the complement – the part after 得 that says how an action turned out.

Mistakes

A verb-like chengyu is already complete: it doesn't take an object after it. The thing involved moves in front.

• × 他半途而废了学习 → 他学习半途而废了

An adjective-like chengyu already carries its degree, so 很 doesn't go in front.

• × 很五颜六色的花 → 五颜六色的花
• × 他的房间很乱七八糟 → 他的房间乱七八糟的

Inside a chengyu the grammar is often classical and can't be taken apart; that is a separate topic.