I wonder whether I need the article in the sentence
"What is [the] statistics on traffic accidents?"
and also what article should I use in the second part of the title of this question.
I wonder whether I need the article in the sentence
"What is [the] statistics on traffic accidents?"
and also what article should I use in the second part of the title of this question.
What are the statistics on traffic accidents? (meaning: Please provide me with the necessary data)
Here, you would need to use the definite article, because the word statistics is postmodified by the prepositional phrase "on traffic accidents" which makes the reference definite.
What is statistics on traffic accidents? (meaning: I don't know what the phrase "statistics on traffic accidents" means)
Here, you would need no article, because you are not trying to get some specific data ("statistics") on some traffic accidents but rather to ascertain the meaning of the whole phrase "statistics on traffic accidents"). It could be that the phrase is used as a column header in a big table, and you need to know what exactly does it reflect.
Such a sentence would, however, need quotation marks to be fully understood in writing:
What is 'statistics on traffic accidents'?
The word statistics is a strange beast in English: a plural noun treated as singular.
Do I need the/an article here?
Since you've used the article the in your first sentence, placing it in square parentheses, the reader is aware what article you are referring to. Hence, it would be okay to use the in the question. In other words:
Do I need to use this article (which I put in the square brackets) here? (such will be the meaning with the)
But you can use an as well: this would imply that you are open to the possibility that it will be a that will suit your example sentence best. In other words:
Do I need to use any article here? (such will be the meaning with an)
Since "statistics" is a plural noun, you need a plural verb. Yes, you need an article before statistics but not before traffic accidents. "What are the statistics on traffic accidents?" would be correct.
"Statistics" is a plural noun; you can usually tell this about English nouns by seeing whether they end with -s and whether removing the -s yields another valid English noun. (Counterexample: princess.)
"Statistics", naturally, is the plural of "statistic". You might equally well ask "What is a statistic [just one statistic] about traffic accidents?", to which I might answer, "130 people died in traffic accidents in Chicago last year." That's certainly a statistic.
If you're asking for "the statistics", that has a connotation of asking for all the statistics that could possibly be relevant (to whatever it is we're talking about), or at least all the statistics that are currently available.
Google tells me that in Russian, the word for a single statistic might be "статистическая величина".
FYI, in English, statistic is also often used as a synonym for casualty (with the connotation of loss or unimportance of personal identity). In your original context, it wouldn't naturally be interpreted that way; but I could write
When her car crashed fatally into that telephone pole, Alice became yet another statistic in America's deadly love affair with the automobile.
This usage usually occurs on its own or with the preposition in, as opposed to the mathematical/statistical usage, which takes the preposition on.
Oh, and I should add: Statistics (as an uncountable mass noun) is the branch of mathematics that deals with statistics (plural). Compare: mathematics, logistics, arithmetic, logic. All of these are mass nouns referring to branches of mathematics. And yes, some of them take -s and some don't; it's pretty much random.
I could construct contexts in which the difference between count and mass nouns was pretty subtle:
Alice studies automobile statistics.
(Probably the count noun is meant: statistics about automobiles.)Beth studies automotive statistics.
(Probably the mass noun: some sub-field of statistics.)Carol studies financial statistics.
(Might plausibly be either one.)
but these aren't very important to your original question. You're unlikely to see anything like these in real life.