noun form such as (My plan is to go to UK)?

[keannu]

There's lots of be-to verbs like the following, and even though only one example represent one meaning, as context always influences the meaning, the meanings can vary. It's really hard to infer the meaning from one of so many possibilities such as (promise, command, intention, etc), so do you have any secret or is context the only solution?
And when you perceive be-to verbs, do you perceive them as descriptive phrase to describe the subject or a noun form such as (My plan is to go to UK)?

1.We are to eat out tonight - promise, schedule
2.You are to finish this by seven - command, order
3.Please make an appointment first if you are to see him - intention, conditional
4.Not a sound was to be heard - possibility
5.He was never to see his wife again - destiny
6.She is to blame for the accident - passive voice

[5jj]

6.She is to blame for the accident - passive voice

No - 'to be blamed' is passive voice. The active 'to blame' has a similar meaning to the passive 'to be blamed' in your sentence, but it is still active voice.

[philo2009]

And when you perceive be-to verbs, do you perceive them as descriptive phrase to describe the subject or a noun form such as (My plan is to go to UK)?

On this purely analytical question, naturally only sense can determine whether an infinitive functioning as copular complement identifies/defines the subject, as in


My plan is to go to the UK.


(N.B. note the article!)


describes it, as in


This was a day to be remembered.


or simply asserts an act or condition relating to it, as in


I am to see him tomorrow.


Thus, the three uses of the infinitive exemplified above would be classified respectively, on the basis of sense, as nominal, adjectival and verbal.

[keannu]

No - 'to be blamed' is passive voice. The active 'to blame' has a similar meaning to the passive 'to be blamed' in your sentence, but it is still active voice.

I thought "to blame" is a short form for "to be blamed", so form-wise it may be active voice, but meaning-wise, it may be passive voice.
My key question was how to select one the so many meaning of "be to verbs", it seems just context as with other cases. And I also wonder why this noun-like forms came to be used as verbs and its origin and the purpose....it's really hard to understand from the Korean perspective....

[5jj]

There's lots of be-to verbs like the following, and even though only one example represent one meaning, as context always influences the meaning, the meanings can vary. It's really hard to infer the meaning from one of so many possibilities such as (promise, command, intention, etc), so do you have any secret or is context the only solution?

BE to, like HAVE to, BE supposed to, other quasi-modal forms and the modals proper, can convey a range of meanings. Context is normally the key.

And when you perceive be-to verbs, do you perceive them as descriptive phrase to describe the subject or a noun form such as (My plan is to go to UK)?

We don't perceive it (not 'them') as either of these. Indeed, unless we are consciously studying it, we don't 'perceive' it as anything at all. If we are studying BE to, then we probably think of it as a quasi auxiliary verb.

I thought "to blame" is a short form for "to be blamed", NO so form-wise it may be active voice, but meaning-wise, it may be passive voice.

It may have a passive meaning, but the form/voice active.

. And I also wonder why this these noun-like forms came to be used as verbs and its origin and the purpose.

I cannot see how BE to can be 'noun-like' It's a verb followed by a to-infinitive (also a verb)

...it's really hard to understand from the Korean perspective.

You'll probably never fully understand any part of English grammr if you look at it from a Korean perspective. The two languages are completely different.