Adverbs like ‘chángcháng’ (often), ‘ǒuěr’ (occasionally) express indefinite frequency; they are often used with nonconclusive situation types (e.g., ‘Tā chángcháng qù hǎibiān’ (He often goes to the seaside), which contrast with conclusive ones in that the latter allow a resultative interpretation of the perfective aspect, as shown ‘Tā kāile mén’ (He opened the door) and ‘Tiān biànle’ (The weather has changed). It is essential to the meaning of ‘kāile mén’, that the door ends up in a new state of being open, which is different from the state in which it started. ‘The weather has changed’ implies that the weather is now better or worse. Therefore, the notions of completion and incompletion apply to the conclusive situation types but not to the nonconclusive ones. For this reason, indefinite frequency adverbs behave differently from the AMC, which usually occur with conclusive situation types and the perfective aspect. Therefore, indefinite frequency adverbs cannot occur after the verb. They behave like definite frequency expressions and are ordered before the VP.
The above reasoning is supported by the fact that indefinite frequency adverbs serve as responses to the question ‘how often?’ or ‘how many times?’ For instance, ‘Nǐ duō cháng shíjiān qù kàn nǐ mǔqin yīcì’ (How often do you go to see your mother?) Such a question would be satisfactorily answered by providing a period of time over which the event in question takes place, like ‘Měi bànnián wǒ qù kàn tā liǎng sān cì’ (Every half year I visit her two or three times) or ‘Chángcháng/Bù cháng/ǒu'ěr qù kànkan tā’ (I often/not often/occasionally go to see her). When people ask a question about the frequency of an occurrence, they are more concerned with frequency with respect to a span of time, rather than with only an absolute number value like ‘liǎng cì’ (twice). So a span of time is always implied even if it is not explicitly specified. Within a span of time, the event described by the predication happens frequently or occasionally. Therefore, indefinite frequency adverbs, like adverbials of definite frequency, are ordered before the VP.
Another explanation we propose is related to the notion of semantic scope. Negative adverbs and certain types of adverbs exert semantic influence upon their neighboring elements. The material that is semantically affected by such an element is in the semantic scope of the element. In general, the material in the scope of a given element should follow the element (Li & Thompson, 1981). Hence, adverbials denoting indefinite frequency of the occurrence of an action always precede the VP, or the VP follows the adverbial so that it lies within the semantic domain of the adverb.
 Loar, J. K. (2011). Chinese syntactic grammar: functional and conceptual principles. New York: Peter Lang.