Why is the 'y' in 'yeniydi' necessary when there is past tense 'di' in place?
When attached to adjectives or nouns, the past suffix has the form -(y)DI. This is due to the older form of the word being idi, which changed over time. :)
How about some grammar notes on past tense?
There are a lot of them in tips and notes.
There is no tips in my turkish course on duolingo
Why "its car" whose is the car ?
It could be his/her/its car according to the ending :)
No English speaker would use the phrase "its car" unless it were owned by a robot.
Duo interchanges his/her/it to incorporate the notion that onun is able to be used for any three scenarios, in other examples the options for he or she will be used as well
Every car was once new...