“潜规则”或“不成文的规定”在英语口语里我们一般说：

Unwritten rule或Unspoken rule,

举俩栗子：

1. You do not tell a girl she's fat, even if she is. It's kinda an unspoken rule.

2. Hey, you can't sleep with your friend's EX! --Why? Is that some sort of unwritten rule?