I wrote ,"It began Monday." Is "on" necessary?
I did the same and agree that "on" may or may not be used in English to express the intent of the German sentence. I reported it. Did you?
I was unsure and did not, but thanks for speaking up.
In German is the short form: Es begann Montags
Would "it started on the monday" be correct?
"It started on the Monday" has exactly the same meaning so why is it makes as being incorrect?
It wouldn't be said (or written) that way. It would just be "on Monday" and not "on the Monday".