"Sometimes there are children playing in the square."
Translation:Soms spelen er kinderen op het plein.
Why cannot it be "Soms spelen er kinderen op plein" just like we can say "er loopt een meisje op straat"?
Good question! No easy answer. There are some Dutch nouns, like straat and tafel, before which you can leave out the article de and just use the preposition op, but unfortunately that doesn't work with plein. I have yet to find information in Dutch about this odd grammar rule.
As Dutchess722, that's a really good question. In general, we always use the article in this kind of construction (op de/het + noun). The case of straat, tafel and school are exceptions. I don't know how many exceptions there are, but you should stick to the normal construction and learn exceptions when you stumble upon them.