不定式和現在分詞均可用于名詞后作定語，但是有區別。其中最基本的區別就是：不定式表示尚未發生的動作，現在分詞表示正在進行的動。如：

1. Do you know the man who will speak at the meeting tomorrow?

=Do you know the man to speak at the meeting tomorrow?

由于定語從句who will speak at the meeting tomorrow用的是將來時態，表示尚未發生動作，所以用不定式來改寫。

2. Do you know the man who is sitting beside your wife?

= Do you know the man sitting beside your wife?

由于定語從句who is sitting beside your wife用的是現在進行時，表示正在進行的動作，所以用現在分詞來改寫。

又如下面的例子：

3. The building which will be built next year is said to be our library.

= The building to be built next year is said to be our library.

由于定語從句which will be built next year用的是將來時態，表示尚未發生動作，所以用不定式來改寫。

4. The building which is being built now is said to be our library.

= The building being built now is said to be our library.

由于定語從句which is being built now用的是現在進行時，表示正在進行的動作，所以用現在分詞來改寫。

由于你的問題沒有寫完整，我們暫且將that you ever did see 視為定語從句，假若它所修飾的先行詞是the man，那么問題就成了the man that you ever did see，它的意思便是：你的確曾經見過的那個人。其中的did在表示強調，又如：He did say you were a thief. 他的確說過你是小偷。I do like the girl. 我的確喜歡這個女孩。He does hope to go abroad. 他的確想出國。

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