麻烦解释下这以下句子为什么要这么做 if we don't want to be late,we'd better( take a taix)

麻烦解释下这以下句子为什么要这么做 if we don't want to be late,we'd better( take a taix)
1.if we don't want to be late,we'd better( take a taix).take a taix可不可以替换成by taix.
为什么?
2.Parts(部分) of the book are interesting.为什么用Parts不能用Part吗?
3.It's about miles from my hone to the supermaket.为什么用miles?能不能用mile或具体数字+mile?
4.最后两个英译汉
Cheshunt is a few miles to the north of London
we had an early supper last evening ,so we took a walk in the park after supper