A word doesn’t necessarily belong to a particular part of speech. Depending on how it is used in a sentence, it can belong to more than one. On is no exception: It can function as preposition, adjective, or adverb.
1. On as preposition
As preposition, it covers range of meanings: that something is above something else and touching it, that someone or something is supported by another person or thing, a means of transport, a day or date, direction, and more. Like all prepositions, it is followed by a noun phrase. Examples:
The dog was sleeping on the rug.
Roger’s birthday falls on 08 August.
I slammed my books on the bed.
2. On as adjective
On as an adjective is mainly used to indicate that something is happening or planned, an equipment is functioning or something is flowing, and something is attached to or covering something. It is mostly used predicatively (only the last example below is an exception), implying it comes after a linking verb.
The field is still wet. Is today’s game on?
I’m on for tonight’s party.
Tom, you’ll be on in five minutes. [Tom will be speaking or performing in five minutes.]
Even though it’s a Saturday, I’ll be on in the pre-noon. [I’ll be working in the pre-noon.]
Such behavior is not on.
Water/ electricity/ gas is on.
The tap/ geyser/ motor/ engine/ TV is on.
The cork is on firmly.
The switch is in on position. [Attributive]
3. On as adverb
On as an adverb is mainly used to indicate that something is being worn or is on somebody’s body; covering, touching or forming part of something; and something is operating or starting to operate.
She put her sweater on.
He turned the lights on.
Can you move on to the next question?
The speaker rambled on and on.
