A small hole which wasn’t mended in time will become a big hole much more difficult to mend.

解释 ( paraphrase )
小洞如果不把它补好，变成大洞就难补了，就要叫苦了。
这条谚语告诉我们：小问题如果不及时处理好，发展成为大的问题就难于解决了。

This proverb tells us that if a trivial problem is not solved in time, it will become a serious and knotty one.

造句 ( example )
有了错误就及时改正，不然“小洞不补，大洞叫苦”，等到想改正的时候就已经来不及了。