Grammar

I really wanted to know why she was a thief.


Q1 : really wanted = ADV形容VERB,絕對正確, 但如用wonder已經有齊"極想了解",一等於三.

Q2 : 一句只用一個原動詞vi/vt,其它均是infinitive/gerund/participle/扮演帶動作性的verb/noun/adj/adv.subject/object/complement.

Q3 : 不是可轉past tense,而是變咗past participle=p.p.=過去分詞,
past tense依然是動詞.

Q4 : ans補充: 是的. 如果有connective是各自各的[原動詞]的.

It is grammatically correct but 'I wondered why she had became a thief.' or 'I wondered the reason for her becoming a thief.' would be better.

If there is no restriction on the time element. The following sentences would be even better.

'I wonder why she has become a thief.' or 'I wondered why she stole [something].'

In a sentence, more than one verbs can be in past tense depending on the situation.

NO. I do not think that there is anything wrong in the sentence.

1. The "wanted" in the sentence describes the time when the person wonders.

2. You can have as many verbs past-tensed as you want, if you use them properly.

:)