Horstmann said after jurors left for the day that he plans to move for a mistrial.
1. 句子主幹：Horstmann said that...
2. that 引導的是賓語從句，從句的結構為：he plans to move for...
V-T/V-I?At a meeting, if you?move?for?something or?move?that?something should happen, you formally suggest it so that everyone present can vote on it. 提議
Somebody needs to move for an adjournment.
需要有人提議休會。
3. after jurors left for the day 是狀語（漢語的習慣是狀語提前）
綜上，全句的譯文是：陪審員們離開後，Horstmann說他計劃申請無效審判。