Did vs was / were

Did/Was/Were Mimi busy last night ?

首先將 question 轉回陳述句 (declarative sentence), 才作選擇

1. Mimi busied last night. ~ wrong
busy 可以做 verb, 但是個 transitive verb, 必須有 direct object
如果 busy 是 intransitive verb, 這個句子是對, 不用有 object
作為動詞, busy 的 past tense 與 past participle 都是 busied
Mimi busied herself last night. ~ 對
Did Mimi busy herself last night ? ~ 對
"busy" 是 main verb. "Did" 是 auxiliary verb ~ used to form question
herself 是 direct object

完全地不同意 Jenkin 的說法, ~ "不是一種"行為"/"動作"(動詞)"
busy (及物動詞) = 忙於

你的孩子用 busy 動詞, 是 busy oneself (有 reflexive) = 使自己忙於
She busied herself in the kitchen last night.

busy oneself with something -
He busied himself with the new camera,

busy oneself doing something
She busied herself putting the clothes away,

Mimi did busy herself in the kitchen last night
"busy" is the main verb. "did" is an auxiliary verb which is used to give emphasis to the main verb.

2. Mimi was busy last night.~ correct
Mimi 是 subject, third person singular
was 是 linking verb, 用 past tense, 因為動作發生在昨晚, singular verb
busy 是 adjective,
是 predicative adjective, 做 subject complement, 形容狀態 (state)
轉回去 question,

Was Mimi busy last night? ~ 這句完全地對

3. Mimi were busy last night. ~ wrong (subject-verb agreement error)
第三句用錯 plural verb "were". 事因 Mimi 是 singular subject

總括:
a) 辨別 linking verb 與 action verb, transitive verb 和 intransitive verb
b) subject - verb agreement
c) busy 可以做 transitive verb, 必需 busy oneself ....不能說 busy something
busy oneself with something, 這 verb 類似 distance (noun, verb)
distance 通常做 noun, 作 verb 要... distance oneself ...

Credit is given to 意見者： 意I思A ( 知識長 ),
他是第一個指出 busy 可做 verb. Thanks.

LAST NIGHT - PAST TENSE

WAS - ONE PERSON OR ONE ITEM
WERE - TWO OR MORE

so you should pick the answer by yourself

Was Mimi busy last night? - busy is used as an adjective
Did Mimi busy herself last night? - "busy herself" means "make herself busy" - busy is used as a verb

2015-05-07 17:01:32 補充：
Busy is a transitive verb that needs a direct object.

用中文講解：

問句是問人物MIMI昨晚是不是BUSY(忙)

"忙"是一種狀態(嚴格查字性是"形容詞")，不是一種"行為"/"動作"(動詞)，
英文文法的提問時只有行動的動作才會用DO/DID
而一個人的狀態/心情/本質等"形容詞"是IS/WAS

另一種方法教她是改為正常句寫出來再變為問句寫:

Mini was busy last night.

假如她真有學英文句子的話，她應該不可能寫成
Mini did busy last night.
或者她應該不會創作出個BUSY的過去式"動詞"吧