Be careful you don't knock your head on this low beam.

小心，别把头撞在这矮梁上。

He stooped under the low beam.

他在低矮的横梁下弯着腰。

I bumped my head on the low beam.

我的头撞到低梁上了。

Mind you don't knock your head (on this low beam).

小心， 别（让这根低梁）撞著头。

Mind you don 't knock your head (on this low beam).

小心，别(让这根低梁)撞着头。