The forest was so dense that sunlight barely reached the ground.

森林如此茂密，阳光几乎无法到达地面。

The fog was so dense that visibility was almost zero.

雾气如此浓密，能见度几乎为零。

He's a bit dense when it comes to understanding complex instructions.

他在理解复杂指令方面有点迟钝。

Only a truly dense person would fall for that trick.

只有真正愚蠢的人才会上那个当。