لَا نَكْذِبُ — We do not lie.
The form نَكْذِبُ still points to “we”; لَا only makes it negative.
لَا يَشْرَبُ الطِّفْلُ الْمَاءَ الْبَارِدَ — The child does not drink the cold water.
First read لَا, then identify the verb, subject, object, and adjective.