1.--I am going to the post office, for I have a letter to post.(active voice, correct)<br/><br/><br/><br/>2.--I am going to the post office. I know you have an important letter to post. Let me post it for you.<br/><br/> --Thank you. But I have no letters to be posted now. (passive voice, correct)<br/><br/><br/><br/>What about this one?<br/><br/>I'm going to the supermarkedt this afternoon. Do you have anything to buy?<br/><br/>Can we say"Do you have anything to be bought"?