Confusion with に、を、と particles used with verbs like 感じる

Question

Consider this sentence:

私は試験のために勉強しなかったことが残念に感じる。I feel regret that I didn't study for the exam.

1.Why is the に particle used instead of と?

I have read the answer to this post which considers how 感じる can be used with the を, と and に particles under different circumstances. But I still have trouble understanding it.

In this part of the answer:

"4. When it appears to take に, the に particle is actually marking an adverb. This happens with transitive and intransitive uses of the verb."

I interpret this to mean that 残念に would be translated as "regrettably" into an adverb. So 残念に感じる means "to feel regrettably". If I translated it literally: "I regrettably feel that I didn't study for the exam."

But why is it also wrong to use と in the sentence above if it is a "defining or quotative" particle?

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Here is my interpretation of the answer to the post but I am not too confident, correct me if I'm wrong (I numbered my questions in this section respectively):

を can only be used in circumstances where the person has a:

• physical sense: e.g. 揺れるのを感じた、風を感じる

• spider sense: e.g. 将来に不安を感じる、彼は生命の危険を感じた

と is used as a "quotation/sound or manner adverbial" phrase

(I have trouble understanding how と particle is used the most)

• Used as a spider sense: 彼は何かを隠していると感じた (2. does this mean that 彼は何かを隠しているのを感じた is also correct?)

3.But what about this sentence? Is に or を also correct in this case?

良書を読むというも幸せと感じる。I feel content in reading good books.

4.What are other cases where you can only use the と particle with 感じる?

に (or i-adjectives + く) is only used as an adverb to express emotional sense

• その時，初めて母の愛を深く感じた。

と cannot be used with verbs that only express a knowledge/forgetting of facts. e.g. 忘れる、思い出す、知る.

But it can be used with verbs that can express feeling or belief e.g. 分かる、信じる、思う、考える

However, considering something from my previous post

私はあなたがいつも忙しいと分かっています。

5.Would it be incorrect to use the adverb rule as stated above?

私はあなたがいつも忙しく分かっています。

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I might have answered my own question already but I am still not too confident of my interpretation. I would appreciate further clarification on how these particles are used with these kinds of verbs, and some examples would be great.

Apologies for the long post, I did my best to keep it as brief and organized as I could. I hope this makes sense.

Answer 1

残念に感じる and 残念(だ)と感じる are basically interchangeable. They both mean "to feel it's regrettable" rather than "to regrettably feel something". This 残念に is "adverbial" in a broad sense, but it does not describe the way 感じる happens. Japanese "adverbial" modifiers do not necessarily work the same way as English adverbs (see this, too). In this case, maybe you can think of this に as a rough equivalent of English "as", e.g., 候補者に選ぶ ("to choose as a candidate"), 教師に採用する ("to recruit as a teacher"), 枕によい ("to serve well as a pillow").

Note that adjectives before に感じる/思う can work in two different ways, depending on the word.

• 奇妙に感じる, 変に感じる, 残念に思う, 遺憾に思う, 悲しく思う, ありがたく思う...
  I feel/think it's strange/pitiful/sad/etc...
• 痛切に感じる, 強く思う, ...
  I strongly/deeply/etc feel/think ...
• よく思う (ambiguous) I think it's good / I often think ...

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• 本を読むと幸せに感じる and 本を読むと幸せと感じる are interchangeable.
• 母の愛を深く感じた only means "I deeply felt mother's love". This 深く is a simple adverbial modifier.
• Please do not overly generalize the use of ～に感じる/～に思う. 忙しく考える only means "to think busily/hard about something". 忙しく分かる/信じる makes almost no sense to me.