Question

“The letter she looked forward to came at last” 请问这为什么用“came”而不用“

“The letter she looked forward to came at last” 请问这为什么用“came”而不用“coming”

Answer 1

The letter(she looked forward to)came at last.
括号里只是定语从句而已，句子主体是the letter came at last,从定语从句可看出是过去时，有looked,所以主句自然也是过去时，永came。

look forward to这里和后面的动词come不是一个层次的句子，前者从句里，后者主句里，切不可套用look forward to doing的句型。

Answer 2

The letter I was looking forward to came at last.
本句有两部分构成：
1.主句：The letter came at last.那封信终于到了
2.定语从句：I was looking forward to (the letter)我正盼望着的（那封信）.
合起来就是：The letter (that)I was looking forward to came at last.（注意,本句是过去时态,所以用came而不用come）
the letter 是先行词,关系代词that或which(代the letter)因为作宾语省略.
翻译：我正盼望着的那封信终于到了.