Do we have to add the article "a" in the below sentence? and why?
"My friend said he is going to be (a) Housing Department officier in next year."
If there's more than one Housing Department Officer in the area, you probably should use the article. If there's only one HDO, the article isn't needed.
"Barack Obama would like to be a President" - You'd only say that if there was more than one president.