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.