非法(fēi fǎ) - illegal, unlawful in Mandarin

非法 (fēi fǎ) is the formal word for illegal. It attaches straight to a noun or verb phrase without 的 in fixed terms: 非法入境 (illegal entry), 非法经营 (illegal trading), 非法收入 (unlawful income). Used as a predicate it takes 的 or an adverb, as in 这是非法的.

The 非 prefix is productive and worth knowing on its own. It negates the word behind it in a formal register: 非正式 (informal), 非官方 (unofficial). Once you see the pattern, a whole shelf of newspaper vocabulary opens up.

The trap is register. 非法 belongs to law reports, contracts and news bulletins. Talking about a mate parked on a double yellow you would say 违法, or just 不行. Its opposite, worth learning in the same breath, is 合法.