Senior Member. When you think about it, it's definitely "write on the blackboard", but with the silliness of prepositions, I wouldn't be surprised or shocked if people frequently said "write in the blackboard" without knowing.May 27, 2009
“For” has multiple uses; it can function as a coordinating conjunction (one of the F.A.N.B.O.Y.S.) to connect words or groups of words together, but it can also function as a preposition.
ON Use on when something is touching the surface of something. It could be a horizontal surface, like a floor or beach, or a vertical surface, like a wall (“They hung pictures ON the wall”). We also use “on” for the surfaces of body parts (“He has a tattoo ON his arm”).
Prepositions and Time English speakers use in to refer to a general, longer period of time, such as months, years, decades, or centuries. For example, we say “in April,” “in 2015” or “in the 21st century.” Moving to shorter, more specific periods of time, we use on to talk about particular days, dates, and holidays .Feb 15, 2018