Can we use şu instead of o?
Yep! It was just a forgotten alternative and it is accepted now.
why can't we say "bu onun sandalyesi"?
That would be "This is her chair".
"It" is usually translated (at least on Duolingo) as "o".
I wrote O onun sandalyesi and was marked wrong. Why is is necessary to have the dir ending? I thought that was optional, or only used for specific reasons. Thanks so much for clarifying!!
I honestly dont understand why some sentences end woth dir.. like mavdir, sandalyesidir etc... How do you know when to add dir and when not to?