I got quite confused these days with the grammar rule that adds 地 to an adjective to make it work as an adverb. My basic understanding is that you must add 地 to make an adjective work as an adverb, while you do NOT need to add it on an adverb, since adverbs can work without any such suffix.
But in the following sentence:
It seems that the word 一个劲儿 is usually used with 地, even if it is adverb (according to Pleco and 现代汉语词典).
Again, my understanding is that an adverb can be used alone while an adjective must be used with 地, in order to work as an adverb. But the rule is invalidated in the sample above.
Another sentence I came across:
According to Pleco, 刻苦 is adjective, not adverb, so you must use 地 in order to use it as an adverb and modify 学习. But this sentence does not use 地.
So what is the rule of 地 on the adjective and adverb?
Is it common to omit 地 on an adjective, even if the adjective works as an adverb?
Does it depend on each adjective that it is OK to omit 地? In other words, you can omit 地 on some adjectives but cannot on others?
Is it common to use 地 on adverbs as well? If that is the case, why do adverbs need 地 in the first place?
Likewise, does it depend on each adverb if it requires 地?