Could anyone tell me the reason which is the correct sentence?

[Farrukhkamran]

1.why did thief leave his bike there?
2.Why the thief would leave his bike?


I understand from above points is that both are in Past tense but I do not understand the usage of 'would' , can someone tell me about usage would?

 

[Raymott]

Neither is correct.
1. "Why did the thief leave his bike there?"
2. "Why would the thief leave his bike?"
Note that all sentences begin with a capital letter.
The second is conditional. It's not past tense.