Under what circumstances can I delete a post?

Recently, I deleted about 50 posts I made on a particular thread. I went to "Edit" and then was given the option of deleting, so I took it.

About two days ago I tried deleting a post on a different thread and was not given that option, so I had to edit it.

A few minutes ago I went to edit a typo I made on a third thread and was again given the option of deleting it.

Some of the posts I deleted had been quoted in other posts, so that is not a factor in stopping deletion.

But I have no idea of when it can be done and when it can't.