一夫当关万夫莫开的英文：
[Literal Meaning]
one/man/guard/pass/ten thousand/man/not/open
If one man guards the pass, ten thousand men are unable to get through.

[解释]
山势又高又险，一个人把着关口，一万个人也打不进来。形容地势十分险要。

[Explanation]
to describe a place which is strategically situated and difficult of access

[例子]
这里的山势非常险要，一夫当关，万夫莫开，我们可以在这里设下埋伏。

[Example]
Here the mountains are strategically located and difficult of access. If one man guards the pass, ten thousand men are unable to get through, so we can set up an ambush here.