Fire can be used as an uncountable noun. When we talk about fire as a ‘thing’ it doesn’t need an article (a, an or the).
“Fire is hot.”
– We are not talking about something burning, we’re just talking about fire itself.
Fire can also be used as a countable noun. When we talk about something that happened fire is countable and needs an article.
“There was a big fire at my neighbor’s house. Many firemen came.”
– This means my neighbor’s house was burning.
“Did you hear about the fire downtown last night? Luckily no one was hurt.”
– Last night a building was burning downtown.
*It can also be a planned event.
“We had a nice fire last night by the lake.”
Phrasal verb – ON fire
-This means something is burning right now.
“My greenhouse is ON fire!”
Phrasal verb – Burn down
-This means burn completely; be destroyed by fire.
“That old cabin in the woods burned down last weekend.”