What is the difference between 还是 and 还? Is 还 the short of 还是 and, consequently, they are interchangeable in every situation, only in some cases or never?
For reference, I will list the different situations I have seen them. Can any be expressed both with 还 and 还是？
means still / despite the situation in a negative sentence：
Subj + 还 + 不 / 没 + [Verb Phrase]. E.g.:
Wǒ jiěshì le zhème duō biàn, nǐ hái bù dǒng?
I've explained it so many times, and you still don't get it?
Subj + Verb， 还 + Verb. E.g.:
Tā yǒu yī gè dìdi, hái yǒu yī gè mèimei.
She has a younger brother and also has a younger sister.
还 + Verb. E.g.:
Zhège diànyǐng hěn hǎo, wǒ kànguò, hái xiǎng kàn.
This movie is very good, I watched it before and want to watch it again.
Subj + 还 + Adj. E.g.:
Wǒ hái hǎo.
I'm pretty/fairly good.
expresses a strong tone of surprise:
Subj + 还 + Predicate + 呢. E.g.:
Hái liúguò xué ne, Yīngyǔ zhème chà.
You have studied abroad, yet your English is so bad.
marks a rhetorical question:
Subj + 还 + [Verb Phrase] + 吗 ?. E.g.:
Tā shuō dehuà, nǐ hái tīng ma?
Did you listen to what he said?
means even more:
Noun 1 + 比 + Noun 2 + 更 / 还 + Adj. E.g.:
Tā bǐ Yáo Míng hái gāo.
He is even taller than Yao Ming.
means still / despite the situation in an affirmative sentence:
⋯⋯ ，(但是 / 可是 +) Subj + 还是 + [Verb Phrase]. E.g.:
Fùmǔ bù tóngyì tā qù, kěshì tā háishì qù le.
His parents didn't agree to it, but he still went.
provides choices in a question:
Option A + 还是 + Option B ?. E.g.:
Nǐ hē chá háishì hē kāfēi?
Do you drink tea or coffee?
means had better:
Subj + 还是 + [Verb Phrase] + 吧. E.g.:
Nǐ háishì kuài diǎn zǒu ba.
You'd better leave now.