You should be able to delete shape elements by right clicking and selecting Delete from the context menu. Note that the rounded rectangle you have added is considered a decoration and won't be filled. To get round rect as shape outline, starting from the initial four line elements of a new shape, delete three of them, for the remaining one select convert to -> round rect from the context menu, and then resize it accordingly.

I hope that helps,
Stoyan

What exactly is the purpose of combine?

Thanks.