虚词"虽然":empty word "although"

The Chinese empty word (虚词) "虽然" or "尽管" corresponds to "although", "though", or one sense of "while" in English. "In spite of" or "despite of" can also use "虽然" as its Chinese equivalent, but "虽然" must be followed by a sentence or at least a verb followed by an object or an adverbial modifier as in "虽然下雨" ("in spite of the rain") or "虽然做完了" ("although [the work] has been completed"), where "下" or "做" is a verb and cannot be omitted.

A basic grammatical difference between Chinese "虽然" and English "although" is that "虽然" strongly calls for "但是" to start the main sentence as in "虽然下雨,但他还是去了" ("Although it rained [In spite of the rain], he went"), while "although" must not have "but"; if you have the urge for it, a "yet" is acceptable.

"但是" here may be considered as a conjunction, but not in the sense that it connects two full independent sentences. In English, two full sentences (with only one period at the very end) must be connected with a conjunction, or a semicolon if the second sentence serves as a further explanation. The Chinese (as well as French) does not have this requirement; the two sentences may be separated by just a comma. Probably due to lack of the requirement for a conjunction between two full sentences in Chinese, the conjunction "但是" in the "虽然...但是..." construct may be omitted, e.g. "虽然下雨,他还是去了".

Because English prohibits "but" at the beginning of the main sentence that has a clause of "although", people bilingual between Chinese and English subconsciously omit "但是" in the "虽然...但是..." construct; to these bilingual speakers, there's no such strong calling for it, or rather, there's a strong calling for not having it.

虚词"当然":empty word "of course"

The Chinese empty word (虚词) "当然" is generally translated as "of course" or "certainly". It makes perfect sense in this example, "你会游泳?", "[我]当然[会] ("You can swim?", "Of course [I can]"). But "当然" is also commonly used in a different context, as in "明天每个人都必须到办公室，当然你事先请假了可以不来"("Everybody must come to office tomorrow. But of course you don't have to come if you asked for leave earlier"). In this case, "当然" is said in a much weaker tone and more resembles "but", "nevertheless", "however" in meaning than the more common "of course". The German "natürlich" ("naturally") or "allerdings" ("though") may be closer to this sense. English does have this meaning, as Wiktionary says "Acknowledges the validity of the associated phrase", e.g. "Of course, there will be a few problems along the way". But this sense is used more often in Chinese.

虚词"很":empty word "very"

The Chinese empty word (虚词) "很" means "very". This translation is straightforward and universally accepted. But there's one little subtlety in its actual usage: "很" is used more often in Chinese than "very" in English. This causes some descriptions using an adjective in Chinese not really "very" much so (if everything is very good, nothing is really that good). For instance, "He's good", "He's good at playing cards" may be translated as "他很好", "他很会打牌"， although they can also be "他不错", "他牌打得[很]好". The sentence "他很好" is not likely to be changed to "他好", which sounds odd, and "他很会打牌" may be misunderstood if shortened to "他会打牌" ("He knows how to play cards"). The apparently superfluous "很" serves no purpose other than making the sentence sound more native. But translators may not realize this and tend to literally translate "很" to "very". This practice seems to be particularly widespread among the translators living in China. I believe the correct way to deal with "很" is to review the context and ignore it if it does not really carry the meaning of "very".

Chinese "empty word" 虚词

The term "empty word", or "虚词", in Chinese, refers to "a word or morpheme that has no lexical meaning and that functions as a grammatical link or marker, rather than as a contentive" according to Dictionary.com. Specifically, they include prepositions, conjunctions, auxiliary words or "Chinese particles", onomatopoeias, interjections, and adverbs[note1]. But in spite of its long history (back to 1890 to 1895, perhaps invented by a missionary or sinologist), the translation "empty word" has the connotation that the words, whoever utters, are not to be trusted, while "虚词" in Chinese is a purely technical, grammatical, term. This makes "empty word" a poor translation for "虚词", although no better one has been proposed. Incidentally, "hollow word", if it were used as a translation, may be closer literally ("hollow" for "虚"), but also has unwanted connotations.

Wikipedia considers the word "expletive" as the equivalent of "虚词". We need to think beyond the more common meaning of "expletive" here (words of profanity), and only consider syntactic expletive and expletive attributive. Because of its common usage of the word, neither is perfect in my opinion. In addition, be aware that an expletive in English is not quite equivalent to a "虚词" in Chinese. The latter is purely based on word class, while grammatical expletives in English are more context-sensitive. That is, all adverbs, prepositions, conjunctions, "Chinese particles", onomatopoeias, and interjections in Chinese are "虚词", with no exception, but there's no such simple rule in English.

Probably because of Wikipedia's English rendering of "虚词" as "expletive", pages of other languages use incorrect or not quite correct words, such as explétif in French, Kraftausdruck in German (words to express strong feelings, swears, expletives), where Formwörter^[note2] or mot-particule^[note3] may be a better term. But the Japanese page uses the Kanji 虚辞.

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[note1] This footnote is needed to avoid simplistic equivalence: English adverbs include almost all words of the construct adjective-ly, but Chinese adverbs are more or less limited to "very", "little", "all", "also", "probably", etc.
[note2] This word may have been coined by German sinologists about a century ago, as in Vergleich der wichtigsten formwörter der chinesischen umgangssprache und der schriftsprache
[note3]> as in Le mot-particule 之 tchē

Off-topic: Learn English to Know China

I read this on weibo.com, the Chinese equivalent of Facebook: "We used to learn English to know the world. We now learn English to know China" (当初我们学英语是为了了解世界，如今我们学英语是为了了解中国). (The earliest occurrence of this quote as of now is on Twitter authored by huqiwen.) The apparent oxymoron in the second sentence is one of the best remarks on the national censorship. Although any web site deemed sufficiently government-unfriendly is blocked, those in English are generally in much better shape than those in Chinese. If this were not so, learning English to know China would be just as ineffective as reading Chinese materials.