Answer:1. We used to live in the city center.
2. School children in the UK have to wear a uniform.
Explanation: In the first sentence, "the" is not needed before "city center" because it is a general reference to a city center and not a specific one.
In the second sentence, "a" is used before "uniform" because it refers to any uniform that school children in the UK have to wear, rather than a specific or previously mentioned uniform.