当春乃发生

From Du Fu's 《春夜喜雨》 (Tang dynasty, ~761), written during his stay in Chengdu: 好雨知时节，当春乃发生。随风潜入夜，润物细无声. A quiet celebration of spring rain that knows exactly when to come; 润物细无声 ('moistening things silently') is now proverbial praise for unobtrusive, nurturing influence.

usage

乃 is classical for 'then / only then'; 发生 here means 'to begin / come forth' (the rain), not the modern sense 'to happen' generally. Always quoted with its opening line 好雨知时节.