主系表后无论是when从句还是that从句都不可能是宾语从句。

Peter was excited when he received a letter from his friend.

这个句子中，when从句为时间状语从句，可以移至句首。was excited与其说是系表结构，不如说是被动语态，即 A letter from his firend excited him. When... he was excited. 句子表示接到信时他的情绪的瞬间变化。

Peter was excited that he received a letter from his friend.

这个句子was excited 为系表结构，excited 为形容词，表示主语的情绪状态（而不是情绪的改变）。that从句为形容词的补足语，表示excited原因。传统语法可以分析为that从句为excited的原因状语，整个excited that he received a letter from his friend为形容词短语作表语。所以that从句不可以移到句首。