This is a classic mongoose question—one of my favourite kind—so-called after the story of a South African farmer who, concerned about an influx of poisonous snakes on his land, tentatively agreed to a Ministry of Agriculture official’s suggestion that he adopt a mongoose (which is very adept at hunting snakes).
The mongoose proved to be very effective—so much so, that his neighbour Adriaan wanted one, too. Since Adriaan’s land was bigger, he decided he needed more than one mongoose, but being unsure of the plural of mongoose (mongeese? mongooses? mongoosen?), he wrote the Ministry of Agriculture official as follows:
Hello. I am Mr. Botha’s neighbour, and have a similar problem with snakes. Please send me a mongoose, too.
P.S. Please send me another one after that.
Logic might suggest that because a bag and a pen represent more than one item, one should say are.
But language doesn’t work by logic—or at least, not that sort of logic. It works by what sounds right.
The listener’s ear doesn’t ‘know’ that after a bag you’re going to mention another object, rendering it a plural proposition. It just knows that Here are a bag sounds wrong, while Here is a bag sounds right. The fact that you throw in another object after that makes no difference—as far as the ear is concerned, it’s just another singular.
(Originally written in reply to a question at Quora.com).