as a noun, 'first' refers to the initial person or thing in a series or the initial stage or part of a process.
The first of the month is always busy at work.
He got an A on his first.
From the first, it was clear that they were perfect for each other.
When used as a noun, 'first' often appears in specific phrases like 'the first of' to indicate the beginning of a period (e.g., month, year).
As an adjective, 'first' is used to describe something that comes before all others in order, time, amount, quality, or importance.
She was the first woman to climb Mount Everest.
The first chapter of the book is boring.
Your first priority should be your health.
When listing items or steps, use 'first' to clearly indicate the starting point. Avoid using 'first' when you mean 'firstly' in formal writing, as in 'First, we must consider the facts.'
as an adverb, 'first' modifies a verb, adjective, or another adverb to indicate that something happens or is done before anything else or before a particular point in time.
She first met him at a conference.
I wake up and first thing I do is check my emails.
Speak first and think later.
When used as an adverb, 'first' can often be interchangeable with 'firstly,' although 'first' is generally considered more natural and less formal. The adverbial use of 'first' often implies not just sequence but also importance.