漢語難易結構“S+難+V”中的?价類情態助動詞“難”産生于先秦時期，來源于形容詞“難”的意動用法。最初，形容詞“難”表意動帶上動詞性賓語時，構成“A+V”結構，可看作不穩定的連動結構，此時“難”的語義特征爲[+感到困難]，表示“某人認爲做某事困難”。當“難”前面主語S由施事擴展到當事或空缺時，“難+V”具有重新分析的可能性，旣可以分析爲“(某人)認爲某事難做”，也可以分析爲“(某事)不容易做”或“不容易與某人做某事”。“難”開始向情態助動詞演變。當主語S進一步擴展到受事時，“難+V”的結構特点與現代漢語難易結構一致，“難”的語義特征爲[+費力]，“難+V”由形容詞表意動語法化爲情態助動詞短語，語義上由“感到做某事困難”演變爲“某事做起來困難”。“難”的語法化過程完成后，在典型結構“S_受事+AV_難+V”的基礎上産生了不同形式的非典型結構。這些情態助動詞短語結構經過后世的發展一直沿用至今。

The valuable modal auxiliary verb of Chinese Tough-Construction, “S+Nan+V”, is originated from the Yidong of adjective “Nan” in the Pre-Qin Period. Primitively, the adjective “Nan” represented Yidong along with verb object to form structure “A+V” which may be considered unsteady serial verb construction. Under the circumstances, the semantic feature of “Nan” means “considering difficult” which expresses that someone thinks it’s difficult to do something. When the subject ahead of “Nan” extended from agent to experiencer or vacancy, “Nan+V” bear the possibility of Reanalysis which stands for either “someone thinks it’s difficult to do something” or “something is hard to do” or “it’s hard to do something with someone”, and then “Nan” initiated the evolution toward modal auxiliary verb. When subject further extended to patient, the features of structure “Nan+V” are in line with the ones of modern Chinese Tough-Construction, and then the semantic features of “Nan” stands for “it takes great efforts to do something”. Furthermore, “Nan+V” converted the Yidong of adjective into the phrase of modal auxiliary verb, and “someone thinks it’s difficult to do something” semantically evolved to “something is hard to do”. After the grammaticalization has been completed, various types of atypical structures have also been generated on the basis of typical structure. The phrase structures of modal auxiliary verb have been evolved and inherited until now.