A stable flame with 安定

Kanji Explanation: Peace and ease with 安

Coco and her friends are trapped in a maze and trying to find a way past a dragon. Seeing the others working out magic to use, Coco tries her hand at a spell, creating a flame. Agott isn’t impressed, but Tetia feels otherwise.

テティア:: 「炎(ほのお)が安定(あんてい)してる！」; “The flame is stable!”; 「前(まえ)はすぐ落(お)ちて燃(も)え尽(つ)きちゃってたのにいつの間(ま)に！？」; “It used to go out and burn out right away, so when did this happen!?”

Key Points

1安定(あんてい) = “stable”

Here, 安定(あんてい)してる means the flame is now burning in a steady, controlled way instead of flickering out.

2Casual progressive: 安定(あんてい)してる

“してる” is the casual spoken form of している.
In this scene, it describes the flame’s current state: 「炎(ほのお)が安定(あんてい)している」 “The flame is stable.”

3Subject marker: 炎(ほのお)が

が marks the flame as the thing showing this state.
The sentence is very short because Tetia is reacting excitedly to what she sees.

4The second line explains why this is surprising

前(まえ)は sets up a contrast with earlier attempts: before, the flame would quickly fail.
落(お)ちて燃(も)え尽(つ)きちゃってた is a casual, compressed way to say it used to die down and burn itself out.
That contrast makes 「安定(あんてい)してる！」 feel like a sudden improvement: “Wait, now it’s stable!?”