洞 + 口 / Noun + 洞
洞 (dòng) means "hole," "cave," or "cavity" and refers to an opening or hollow space in a surface or structure. It is used in both literal contexts (a hole in a wall) and figurative ones, and often appears with 口 to form 洞口 (the opening of a hole).
墙上有一个洞,风一直从那儿吹进来。
qiáng shàng yǒu yī gè dòng, fēng yīzhí cóng nàr chuī jìnlái.
There is a hole in the wall, and wind keeps blowing in through it.
小兔子躲进了树下的洞里。
xiǎo tùzi duǒ jìn le shù xià de dòng lǐ.
The little rabbit hid in a hole under the tree.
他们在山里发现了一个很深的洞。
tāmen zài shān lǐ fāxiàn le yī gè hěn shēn de dòng.
They discovered a very deep hole in the mountains.