if not better than 是插入语
所以可以放在一边，不要理他
句子就变成 John plays football as well as David.
可以这样考虑，“if not better than”是插入语，在语法上可有可无，去掉后就是John plays football as well as David. John足球踢得和David一样好。
If not better than, John plays football as well as David.
如果不比David好，那John的足球踢得至少和David一样棒。
as well as 和...一样好。句子的意思是john即使踢球没有David好，也和david踢的一样好。
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