inform against/onsb
—phrasal verbwithinformverb[T]uk/ɪnˈfɔːm/us/ɪnˈfɔːrm/
If youinformon/against someone, you give thepoliceinformation, usually secretly, about thatperson,showingthat he or she has done somethingwrong.
（向警方）告发（某人）
Theterroristssaid that anyonecaughtinformingon them would bekilled.恐怖分子说，举报他们的人一经发现将性命不保。