The letter I was looking forward to came at last.为什么用came不用

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。