1、“落后”在英文中可以这么写：fall behind；lag behind；backward。

2、例句：

（1）“落在后面”：fall behind；lag behind

Be modest and prudent，never lag behind！谦虚谨慎，永不落后！

Put on a spurt，or you'll fall behind。加把劲，不然你要落后了。

（2）“不先进”：backward

Put an end to the backwardness of the mountainous areas。改变山区落后面貌.