Student or Learner
I have learned that a direct object can't be placed after an indirect object in a sentence using verbs such as tell, send, etc. which require two objects(ID, DO).
So I think "I don't know how to tell you this" is not correct. But this sentence seems available. Could explain this to me?
It is colloquial and common, even if not strictly grammatical. In formal assignments, keep to the grammatical structure, but feel free to use the colloquial form in speech.