Is the usage dash "--" okay in formal writing?

Oops! I guess I should have responded here, eh? I am flattered that you still have me listed in your signature, Oristar.

In the context of that particular passage, oristarA, it would be better not to use dashes at all. It looks awkward. Just end the sentence '...reacted at once.' with a period. The fellowing quote is, after all, a new thought.

As a general rule, however, one should use a double dash in the type of situation you have mind when the intent is to make the reader pause for just a moment. Example: He reached into the bag and came up with -- nothing! The dash here provides an element of surprise. The most common place that these double dashes are used is to set apart something that doesn't go comfortably within commas or perentheses. Example: He was pretty sure -- but not completely certain -- that this was the right way to go. (Although in that last example commas would probably work just as well.)

Hi Mr.Micawber, your English is so excellent! That is why I've naturally been keeping your name in my signature.

Well, Merry Andrew, I rewrite that like this:

and he reacted at once, "Chen Shengshen? A great mathematician, wonderful! Every Tianjinese who has ears knows him! What a talent he is!"

Is this fine now?

Another question is:

If what the driver said in the quotation marks is just part of what he said at that time, could I remove the quotation marks? That is, can it be:

and he reacted at once -- Chen Shengshen? A great mathematician, wonderful! Every Tianjinese who has ears knows him! What a talent he is!

No, the quotation marks must stay. Any time you are quoting the exact words one says, they must go within quotation marks. The exception would be if you are paraphrasing, not using the exact words. For example: He reacted at once. He said that Cheng Shengshen was a great mathematician and that every Tianjinese with ears to hear knew of him. "What a talent he is," he exclaimed.

Notice that the quotation marks are still used where the quotation is exact.

Thank you MA.

Another question:

"Every Tianjinese with ears to hear knew of him" is an improper exaggeration, right? Because babies all have ears and are impossible to know a mathematician. So I rewrite it as:

"Every Tianjinese with head to think knew of him".

Do you think the rewriting is proper?

Yes, probably the second version is more exact. However, as an idiomatic statement, there is nothing wrong with the "ears to hear" version either. It's a poetic way, rather than an exact way, of phrasing it.

Thanks MA.

PS. Do you think using 'In the background of the photo" as subject is proper?

Context:

In the background of the photo was a wall, with a round quartz clock hanging there, which clearly showed the moment: 1:13:04PM, April 5, 2002.

The wall is the subject in that sentence, not the background of the photo.

Anyway, may I sugest you phrase that as "A round quartz clock, hanging on a wall in the background of the photo, clearly read 1:13:04PM, April 5, 2002." ?

An inversion?

Either sentence works, but I think mine sounds more natural. Alternatively "Hanging on a wall in the background of the photo, a round quartz clock clearly read 1:13:04PM, April 5, 2002." would work too.

I just think "with a round quartz clock hanging there" sounds awkward.