Tom has been in China for three years.(改为同义句)

1.Tom has been in China (since ) three years(ago ).
2.Three years ( have ) ( passed ) ( since) Tom cane to China.这里的year少了s哦
3.Tom ( came ) ( to ) China three years (ago ).
4.It is (three ) ( years) (since ) Tom came to China.
再问： 第一题有了since还能用ago 吗？还有一题 It's five years since I taught maths 改为同义句 I ( )taught maths ()five years I ()()()maths 5 years ago
再答： 可以的哦 It's five years since I taught maths 改为同义句 I (have )taught maths (for)five years I (began)(to)(teach)maths 5 years ago