【答案解析】Duality refers to the fact that in all languages so far investigated there are two levels of structures or patterning. At the first, higher level, language is analyzed in terms of combinations of meaningful units (such as morphemes, words etc.); at the second, lower level, it is seen as a sequence of segments which lack any meaning in themselves, but which combine to form units of meaning. For example, a syllable is the smallest unit that is normally spoken by itself, and scores of syllables become the carriers of hundreds of meaningful segments of words that are called morphemes.

【答案解析】A synchronic description takes a fixed instant (usually, but not necessarily, the present) as its point of observation, and it describes a language at some point of time, as if it stopped developing. In contrast, diachronic description is the study of a language through the course of its history; therefore, it is also called historical linguistics.

【答案解析】The distinctive feature refers to a property which distinguishes one phoneme from another. For example, "voicing" is a distinctive feature, since it plays an important role in distinguishing obstruents in English.

【答案解析】Semantic field is sometimes also called lexical field. It refers to an organizational principle that the lexicon and groups of words in the lexicon can be semantically related in such a way that they are from the same area of knowledge or interest; and there is a meaning inclusion relation between the items in the field and the field category itself. For example, the semantic field of agriculture includes: farm, farming, tractor, meadow, crop, and so on.

【答案解析】Cooperative principle was proposed and formulated by Grice. It expresses that in making a conversation, the participants must first of all be willing to cooperate; otherwise, it would not be possible for them to carry on the talk. It goes like this: "make your conversational contribution such as is required, at the stage at which it occurs, by the accepted purpose or direction of the talk exchange in which you are engaged". This principle can be further specified as the four following maxims: Quantity, Quality, Relation; Manner.