Recently, I've heard this sort of theory so I'm gonna share this with you and want to hear your opinions as well.

It's about categorizing the language into "Objective or Subjective (not she, her ..)" depending on the meaning.

1. Objective meaning : do (perfect tense included), auxiliary verbs (not subjunctive), relative-clause, conjunction-clause. 

2. Subjective meaning (In speaker's mind, personal, emotional) : to do (yet happened) , ~ing (now), ~ed (happened).

For example ...

[1] do Vs. ~ing; -ed

- The book sells well. (The fact that many customers buy this book. objective)

- The book is selling well. (From the speaker's judging and opinion and thought in his mind, many customers buy this book now, subjective)

- The book is well sold. (From the speaker's judging and opinion and thought in his mind, many customers bought this book. subjective)

- He has gone. (Indicates the fact that he left. obj)

- He is going. (In his mind, he is going now. subj)

- He is gone. (Judging from his thought, he left. subj)

- I'm dying to drink water. (In the speaker's mind, emotional meaning)

cf) I die to drink water. (literally die, obj)

- He's being kind. / She's always crying. / I'm lovin' it! (Based on the speaker's thought, emotion)

cf) He's kind. / She always cries. (obj. but you can't cry always) / I love it.

- I invited you. (Fact)

- You're invited. (So please come - additional emotional meaning. subj)

[2] auxiliary verbs Vs. to do

- I will kill you. (I plan to kill you. obj)

- I'm to kill you. (I want to kill you. I could, would, might, should kill you. ... subj)

- If I'm to be killed a hundred times, ~ (In the speaker's mind. subj)

cf) If I'll be killed a hundred times, ~ (obj. but you can't be killed a hundred times.)

[3] phrase (to do, ~ing, ~ed) Vs. clause (relative, conjunction)

- I'm glad that I meet you. (I'm glad for the fact that I meet you. obj)

- I'm glad to meet you. (I'm glad for the personal reason. subj)

- If you do so, you would feel better. (if-clause, from your point of view, objective)

- Doing so, you would feel better. (From my point of view = personal, subjective)

- Birds singing in the morning, would sing for hunger. (In the speaker's mind, hypothetical birds, subj)

- Birds that sing in the morning, (fact, obj.)

Thank you for reading a little bit long post. I wonder if you've heard this before and we can generalize this?