Understanding these different phrases requires a basic understanding of grammar and some etymology.
The phrase “You are welcome” is the response given when someone thanks us. In this situation, “welcome” is functioning as noun (predicate nominative) and conveys the meaning of “something pleasurable that is invited.”
Basically, when we say “You are welcome” in response to a thank you, we are telling the person that he was a pleasure to help and that he is free to ask for help again.
- i.e. “Thank you for your help.” / “You are welcome!”
Moving on to the phrase “You are welcomed” we see that the word welcome is in its past participle form. In this situation, it is functioning as an adjective and means that you are greeted or accepted into a location or situation.
- i.e. When you arrive at the hotel, you are welcomed with a fresh, hot pastry and a cup of coffee.
The last phrase “You welcome” has the word “welcome” acting as an action verb. This conveys the meaning that you are doing the action of welcoming someone (or something) else.
- i.e. When you welcome a guest into your home, do so with friendliness and grace.
In summary, you must determine if you are replying to someone’s thankfulness, describing the acceptance of something or actually doing the action itself. The confusion occurs when using the subject “you”, but not with other subjects such as I, he or we.