ДЕРЖАТЬ УХО ВОСТРО (coll., human.); ДЕРЖАТЬ УШКИ НА МАКУШКЕ

(coll., usually humor, human) with нужно, надо, следует

1. • с кем not to trust someone, to be very cautious with someone: be on your guard with someone
   • be very careful with someone
   • watch someone carefully
   • watch one’s step with someone
2. • to be cautious, attentive, vigilant (in order to avoid danger or trouble, protect one’s interests etc): one must (has to) be on his guard (on the alert)
   • one must keep his eyes and ears open
   • one must (has to) keep his ears to the ground (keep his ears pricked)
   • one must keep a sharp lookout