"How can X be" vs. "how does X can be"

I have a Spanish friend, who wrote the following sentence:

"How does foo, bar, baz can be compared?"

I corrected it to read:

"How can foo, bar, baz be compared?"

Other than the obvious, he has asked me to explain why the first is wrong, and the second (maybe) is correct.

Any help?

Solution 1:

The phrase "how does ...?" is singular, so the reader will always expect a single entity to follow. For example: "How does magnetism work?" or "How does a bird fly?"

The plural version would be "how do ...?" as in: "How do flowers grow?" or "How do John and Mary know each-other?"

In either case, using the verb "do" implies that there is a single, correct answer to the question. In your sentence (unless I have misinterpreted it), the intent is to ask a more open-ended question. This meaning is expressed by using "can" instead of "do".

To illustrate, if you were to ask: "How do foo, bar and baz compare?" A single answer is expected. As in: "Foo is better than bar, but baz is the best because ...". Asking the question using "can" instead of "do" allows the reader more freedom in answering. For example: "How can foo, bar and baz be compared?" "Foo, bar and baz can be compared using the Kwyjibo algorithm, or by simply comparing their relative weights."

On a side note, I would always use "and" between the last two items in a list: "foo, bar and baz" instead of simply "foo, bar, baz". I don't know if there is a strict rule for such lists, but I find that the "and" reads better because I would always say it that way if I were speaking instead of writing.

Solution 2:

In modern English "do" is obligatory for forming negations and questions, unless there is an auxiliary verb (or the substantive verb is "be", or for some speakers "have").

A passive is formed with the auxiliary "be", so it is not negated with "do":

"A, B and C are compared"
"How are A, B and C compared?" (Not "How do A, B and C be compared?")

If another auxiliary, such as "can" is used, the same thing happens, whether or not there is also a passive auxiliary:

"A, B and C can be compared"
"How can A, B and C be compared?" (not "How does A, B and C can be compared?")

On the other hand, it is common in speech to express passive meaning by the word "get", as in

 "A, B and C get compared"

but "get" does not behave like an auxiliary, and does require "do":

 "Do A, B and C get compared?" (not "Get A, B and C compared?").