Breakdown of senshitsu ga chigau.

Questions & Answers about senshitsu ga chigau.
が marks 泉質 as the thing that is different.
With 違う, the thing that differs is often marked by が:
- 味が違う = The taste is different.
- 色が違う = The color is different.
- 泉質が違う = The spring quality/composition is different.
If you used は instead, the nuance would change. は would make 泉質 the topic, often with a contrastive feeling:
- 泉質は違う。 = As for the spring quality, it’s different.
So が is the most neutral way to say that the spring quality itself is different.
Even though English uses the adjective different, Japanese uses 違う, which is a verb.
That is why it conjugates like a verb:
- dictionary form: 違う
- polite form: 違います
- past: 違った
- negative: 違わない
So 泉質が違う is grammatically more like The spring quality differs than The spring quality is different, even though the natural English translation may use different.