Two lines from 三字經 are 爾男子，當自警.
I understand 爾男子 as "you boys" as if asking for attention by adressing them. 警 is to warn or to guard or being in a state of attentive observation. Now as for 當 and 自 I am very unsure, what is their function in this sentence , how would you explain their grammatical roles in terms of English grammar (adverbs, particles,sth. else ? )