The act of compiling could either be 'compiling', or 'compilation'. Although both are nouns, and either would be correct, I prefer to use 'compiling' for the process, and compilation for the result.
In the sentence, "Compiling takes a long time.", not only is 'compiling' a noun, it is the subject of the sentence. 'Takes' is the verb.
Verbs are neither passive nor active in form; voice is either passive or active.
==> It is compiled : present tense of "to be compiled"
No, the verb is 'is' and 'compiled' is a predicate adjective.
If the sentence were "It is to be compiled", then 'is' is still the verb, but the phrase 'to be compiled' is an adverb infinitive.
==> It has been compiled : finite past tense of "to be compiled"
Not quite. A finite verb is one that stands alone in the predicate. The sentence "It has been compiled" does not have a finite verb; it has a verb phrase ending the past participle of the verb 'to compile'. The tense of that sentence is past perfect.
==> I fail to see why 'compiled' cannot both be a verb and an adjective.
I compiled the code. ==> verb
The compiled code is in that directory. ==> adjective
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