When should I use 地 with an adjective and when should I not use it with an adverb (and why do some adverbs require 地)? 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 to an adverb, since adverbs can work without any such suffix.
But in the following sentence:

他一个劲儿地问，我只好告诉她了。
邻居在装修房子，从早到晚都在一个劲儿地敲。

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:

王羲之从小刻苦学习书法，他的书法艺术吸收前任书法名家的长处，又有自己的发挥和创造，达到了书法艺术的高峰。

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 with an adjective, even if the adjective works as an adverb?

Does it depend on each adjective whether 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 with 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?
一个劲儿地 is not a standard adverb, It is an adverbial phrase
一个劲儿 (constant effort) is a noun. Adding 地 make it an adverbial phrase 一个劲儿地 (with constant effort)
刻苦 is not only an adjective, it can also be an adverb or noun

Is it common to omit 地 on an adjective, even if the adjective works as an adverb?


強烈反對 could mean 'strong objection [強烈 (adj) 反對 (n)] or 'strongly object' [強烈 (adv ) 反對 (v)] .

強烈地反對 could only mean 'strongly object'. Without another context, you cannot omit 地 when you meant to use it as an adverb


You can only omit 地 when the context clearly indicated 強烈 is an adverb, not an adjective. For example, 他強烈地反對 (he strongly objects) can be reduced to 他強烈反對

猛烈攻擊 could mean 'fierce (adj) attack (n)' or 'fiercely (adv) attack (v)'

猛烈地攻擊 could only mean 'fiercely attack'. Without another context, you cannot omit 地 when you meant to use it as an adverb


You can only omit 地 when the context clearly indicated 猛烈 is an adverb, not an adjective. For example: '對他猛烈地攻擊' (fiercely attack him) can be reduced to '對他猛烈攻擊'

Does it depends on each adjective that it is OK to omit 地? In other words, you can omit 地 on some adjectives but cannot on others?

See the examples above. Notice, an adjective that is exclusively for nouns cannot be turned into an adverb by adding 地. For example 強大，細小，好，壞

Is it common to use 地 on adverbs as well? If that is the case, why do adverbs need 地 in the first place?


Adverb with 地 emphasizes 'the manner' of the action. For example '大力地打'

Adverb without 地 focus on the verb itself, with the adverb as an equal-weighted complement. For example '大力打'

Single character adverbs usually don't use 地. For example '痛打'，'狠批' (never '痛地打'，'狠地批')



Likewise does it depend on each adverb if it requires 地?

Yes, usually adverb for verb can be used with or without 地. (see example above).  Adverb for adjective does not need 地, E.g. 局部成功,  絶頂聰明 (never 局部地成功, 絶頂地聰明)