The potential complement and the complement of degree may sometimes look identical and cause confusion to non-native speakers of Chinese. The following two sentences are taken out of context. They look the same, but they could mean very different things. In sentence A, the stress is on the complement 'hǎo好 (well) '. In sentence B, the stress is on the predicate verb 'chàng唱 (to sing) '. Because the complement of degree focuses on the quality of an action, while the potential complement focuses on whether the action can be carried out, the complement in sentence A is a degree complement while the one in sentence B is potential. It would be hard to tell the difference if one read these sentences in characters and out of context. In real conversation, there is no confusion because of the context and the stress put on different words. Please see the following examples. Words highlighted in yellow are stressed.
Tā chàng de hǎo ma? (degree complement; note there is space between the verb and the complement.) 她唱得好吗？ Does she sing well?
Tā chàng de hǎo ma? (potential complement; note that there is no space between the verb and the complement) 她唱得好吗？ Can she sing well?
A closer examination of these two complements reveals even more differences. The following chart shows their different forms. Highlighted words are stressed when spoken.
xiě Hànzì xiě de hěn hǎo 写汉字写得很好
...write characters well
xiě de hǎo Hànzì  写得好汉字
...can write characters well
The complement of degree is an assessment of an action or state. Therefore, the stress is on the complement, which comments on the action or state. The complement can be modified by adverbs such as hěn很, fēicháng非常, tèbié特别.
Tā xiě de fēicháng hǎo. 他写得非常好。 He writes very well.
The potential complement expresses possibility. Therefore, the stress is on the action itself. This is why the predicate verb is stressed. No adverbs can modify the complement.
Tā xiědehǎo. 他写得好。 He can write well.
The negative form of the complement of degree keeps the particle de得 and negates the stative verb with bù不. The stress is on the negative adverb and the complement.
Tā xiě de bù hǎo. 他写得不好。 He writes badly.
The negative form of the potential complement replaces the particle de得 with bù不. The stress is on the predicate verb.
Tā xiěbùhǎo. 他写不好。 He can't write well.
The complement of degree and the potential complement look the same in ma吗 question form, but the stress falls on different elements when spoken. Compare the following two sentences.
The complement of degree:
Tā xiě de hǎo ma? 他写得好吗？ Does he write well?
The potential complement:
Tā xiědehǎo ma? 他写得好吗？ Can he write well?
The complement of degree and the potential complement have different choice-type question forms. The complement of degree forms a choice-type question by juxtaposing the affirmative and negative forms of the stative verb; the potential complement forms a choice-type question by juxtaposing the affirmative and negative forms of the predicate verb and its complement.
The complement of degree:
Tā xiě de hǎo bù hǎo? 他写得好不好？ Does he write well?
The potential complement:
Tā xiědehǎo xiěbùhǎo? 他写得好写不好？ Can he write well?
A verb with a potential complement can take an object, but the degree complement cannot be followed by the object of the verb.
The potential complement:
Tā xiěbùhǎo Hànzì. 他写不好汉字。 He can't write characters well.
The complement of degree:
Tā xiě de bù hǎo Hànzì. (ungrammatical) (The complement of degree can't take an object.) 他写得不好汉字。 He does not write characters well. Tā xiě Hànzì xiě de bù hǎo. (grammatical) 他写汉字写得不好。 He writes characters badly.
The complement of degree can be used in bǎ把 - sentences, while the potential complement can't.
Tā bǎ dìtú guà de gāogāode. 他把地图挂得高高的。 He hung the map high up.