Yes, usually you can share your post to your story. Most social media platforms have this functionality. However, it may depend on the privacy settings of your post and the specific rules of the platform.
In many cases, yes. However, make sure the post is publicly available and that the platform you're on supports sharing to stories. Also, check if there are any specific conditions or limitations set by the owner of the post.
No, Twitter doesn't have a traditional 'story' feature like some other social media platforms, so you can't share your post to a story in the same way.
No. Generally, a private post is private for a reason. It's not designed to be shared widely like on a story. A private post often has restricted access for a select group or just the user, and sharing it to a story, which is usually more public - facing or at least accessible to a wider audience, goes against the concept of privacy.