合法(hé fǎ) - legal, lawful in Mandarin

合法 (hé fǎ) means legal in the sense of permitted by law. It works as a predicate on its own, 这样做合法, and takes 的 in front of a noun, 合法的工作许可. Its negative is 不合法 or the stronger 非法, which is the word you want for illegal in headlines and on signs.

It belongs to a neat family built on 合: 合理 reasonable, 合适 suitable, 合格 up to standard, 符合 to conform to. In each of them 合 carries the idea of fitting a standard, and once you see that the whole set becomes easy to guess.

The trap is English legal being two words in one. When you mean relating to the law, as in legal advice or the legal profession, Chinese uses 法律: 法律意见, 法律问题. 合法 only ever means lawful as opposed to unlawful, so 合法问题 does not mean a legal question; it means a question about whether something is allowed.