Which vs. That.
Use “which” to describe nonessential information—set apart by commas.
- The phone, which Dan never liked, fell into the hot soup.
- The XYZ Project, which took months to complete, won an award.
Use “that” to describe essential information.
- Dan scooped out the phone that fell into the hot soup.
- The project that won the award took months to complete.
Use “who” for people:
- The CEO who endorsed the project retired.