In a way that is not allowed by law. It is the adverb of 'unlawful', and it carries the same official, legal flavour: you meet it in charges, verdicts, and formal accusations rather than in conversation. 'He was unlawfully detained for eleven hours.' 'The company unlawfully withheld their wages.' 'She entered the property unlawfully.' Because it modifies the verb, it is the natural choice when you want to say that the doing of something broke the law — not that the thing itself is a forbidden object. Compare: 'illegal weapons' (the things are forbidden) with 'the weapons were obtained unlawfully' (the getting broke the law). English lawyers use it constantly in that second pattern: obtained, held, dismissed, evicted, arrested, killed — all unlawfully. In everyday speech a Taiwanese student should say 'illegally', or simply 'against the law'. 'Illegally' is the word you will hear from a friend; 'unlawfully' is the word you will read in a judgment or a police statement. Neither is more correct — they are aimed at different readers. Watch the sentence rhythm: it usually comes right after the verb or between the auxiliary and the main verb — 'was unlawfully arrested', 'acted unlawfully'. 'The minister acted unlawfully' is a complete, very English accusation, and one of the commonest ways the word appears.

違法地、不合法地（法律／新聞正式用語）；日常口語說 illegally 即可