Yes, the word it can be the subject of a sentence.
It is hot outside today.
It hasn't rained this hard in ten years!
What is the subject of this sentence? She was the subject of an investigation.
This is too bad. This is the subject in this sentence.
This is too bad. This is the subject in this sentence.
A sentence using the word subject: My favorite subject in school was Science.
In the sentence 'Can you give a definition for that word?', YOU is the subject.
the subject is what the sentence is about
Subject
The word 'is' can begin a sentence but not be a question when it is the subject of the question. Example sentence - Is can be the subject in a sentence. But only in sentences of that type (above) or, 'Is' is a two letter word. Otherwise is cannot begin a sentence.
the simple subject of a sentence is what the sentence is in one word
Yes.
No.
In a sentence, the subject is the word that tells what it is about. In the sentence "watching the light turn yellow" the subject would be the word watching.