Is there a difference between starting a question with "How about" and "What about"? Can we use both expressions interchangeably?
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I think they can be used interchangeably, there is a lot of overlap between the two, but I would usually use
What about can also express an objection, whereas how about does not.
From what I´ve learned teaching English, and what I´ve seen in most books, the difference is that we use WHAT ABOUT + noun, and HOW ABOUT + verb. It´s a pretty simple explanation, but that´s how it´s being taught in schools in Brazil.
This is a very good way to describe it. Here are some examples:
Andy: "I'm bored. There's nothing to do." Danny: "How about we go see a movie?"
Andy: "I'm bored. Let's go see a movie." Danny: "What about our exams tomorrow?"
In the first example, Danny expands the possibilities: Andy thought there was nothing to do, but Danny reminded him that they could see a movie.
In the second example, Danny limited the possibilities: Andy wanted to see a movie, but Danny reminded him that they had to study for their exams.
On a side note, "what about" cannot take a sentence. "What about we see a movie?" is wrong. You can say, however "What about seeing a movie?" or "What about a movie?"
I think how about is used when suggesting an idea, which could be answered or not, whereas what about requires answering, but I'm not very sure..