Hey Medha! Aldehydes doesn't exist in secondary forms since it a terminal a functional group. Basically, the position of characteristic carbonyl bond (C=O) in the parent chain differentiate between aldehydes and ketones. At terminal position, carbonyl carbon makes two bonds, one with the hydrogen and other with the parent chain carbon. If this carbonyl bond is situated anywhere in between the parent chain it can maximum be connected with two carbon atoms and will be called a ketone.
Based on this information, if you try to make an aldehyde into secondary aldehyde it will remain no longer be an aldehyde and become a ketone itself :-)
Hope this makes sense!