1. Water freezes at zero degree.
2. Water freezes at zero degrees.
[Which one is correct?]
3. My best friend has the new cellphone.
4. My best friend has a new cellphone.
[What is the difference between #3 and #4? Why is 'the new cellphone' used in #3?]
2 is correct. You'd use 1 only if it was one degree.
4 is better. He has one cell phone, right? You'd use "the" only if you were referring to the very latest one on the market, such as iPhone XR.