得 / 不 + 补语
dé / bù + bǔ yǔ
Potential complements
Slipping 得 or 不 into a complement says whether the action can be completed at all.
Verb + 得 / 不 + Complement
How it works
做得完 (can finish it) versus 做不完 (can't). The 不 version is far more common in speech.
This is different from 不能做完, which is about permission or circumstances. The potential complement is about inherent possibility: there isn't time, you can't reach, it won't fit.
Very common ones: 听不懂, 看不见, 买不到, 吃不下, 想不起来.
Examples
jīn tiān de gōng zuò wǒ zuò bù wán
今天的工作我做不完。
I can't finish today's work.
wǒ kàn bú jiàn hēi bǎn
我看不见黑板。
I can't see the blackboard.
zhè ge dōng xi mǎi bú dào
这个东西买不到。
You can't get hold of this.