words ending in -tion and -sion are nouns and not gerunds. Most verbs ending in -ate and -ize and many others have a related noun. But there is no strict rule. The word ending in -tion is a word with related meaning:
So consider "to inform" This means "tell somebody something" I can use it as a gerund:
Informing your boss of any ...
after to be verbs we use an adjective
No, not necessarily. "To be" can be followed by a noun:
I am an engineer.
You are a genius.
They are librarians.
There is nothing wrong with "humans are omnivores" or "humans have traditionally been omnivores".
If we use the term our usual way, yes, there is one sky.
Cambridge Dictionary, however, also has this definition:
skies [ plural ]
the sky in a particular state or place:
For weeks we had cloudless blue skies.
We're off to the sunny skies of Florida.
Ngram shows also significant use of 'skies'.
We may deduce that it is countable.
You understand that "romantic" can be both an adjective (eg "he is romantic") and a noun (eg "he is a romantic").
The expression "the [noun] in me" is quite common and can actually work with either an adjective or a noun!
With an adjective, the expression is quite literal. For example, someone of mixed heritage might ...
An appositive noun or noun phrase follows another noun or noun phrase in apposition to it; that is, it provides information that further identifies or defines it. Such “bonus facts” are framed by commas unless the appositive is restrictive (i.e., provides essential information about the noun).
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