fire is strong..

You need "The fire ...". All of the adjectives are possible apart from "hard", but these may not be massively natural things to say in real-life conversation. People tend not to speak in such bald statements. You might say, e.g.:

The fire looks as if it's going out.
The fire's nearly burned out now.
The fire's burning well, isn't it?

Thank you a lot~
I appreciate it~

@GPY, why do we need the definite article with fire? Because we are talking about specific fire?

That depends on the meaning. If JUNHEE LEE really is talking about the fire right in front of him then he would need to say "The fire"
If he is talking about fire in general, maybe in a poetic sense, he should leave "the" out.
Here's another example with another word:
Water is the source of life.
The water is really cold.

Fairy Princess@GPY, why do we need the definite article with fire? Because we are talking about specific fire?

In the context of the orginal sentence, yes. Without the article, "fire" would be uncountable, and would refer to the phenomenon in general. For example, from the Wikipedia article: "Fire is the rapid oxidation of a material in the exothermic chemical

You are speaking about the character of fire, in general.

Fire is hot.
Fire can burn you.
Fire has colors. The hottest fire is blue and the coolest is yellow, with red in between.

When you are talking about a specific instance of fire, use "the" or "a":

The campfire is hot. We can roast marshmallows.
The fire is very hot. Don't get too close.
Let's make

Thanks @GPY and @AlpheccaStar.