Vincent Teo Can I say, (a) She is writing some words in / on the blackboard. (b) She is writing on the whiteboard.
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Vincent TeoCan I say,
(a) She is writing some wordsin /on the blackboard.
(b) She is writing on the whiteboard.
Vincent TeoCan ISay,"At the blackboard" is where he is standing. (b) is okay, but I prefer (a). This is American use. In other places, perhaps "at the blackboard" is more common.
(a) He is writing on the blackboard.
(b) He is writing some letters at the board.