请将下列文字翻译成中文，要求语言风格符合翻译学论文要求，简洁有力：Shift represents some changes occurring in a translation process. Translation shifts occur both at the lower level of language, i.e. the lexicogrammar, and at the higher thematic level of text. Catford (1978: 73) states that by shift we mean the departure from formal correspondence in the process of going from the source language to the target language. Further, he states that basically, in shift of translation, or transposition he says, it is only the form that is changed. In addition, he urges the translation shift is done to get the natural equivalent of the source text message into the target text (1978: 76). Translation shifts also occur when there is no formal correspondence to the syntactic item to be translated (Machali, 1998: 3). According to Bell (1991: 33), to shift from one language to another is, by definition, to alter the forms.
Catford (1978) divides the shift in translation into two major types, level/rank shift and category shift. Level/rank shift refers to a source language item at one linguistic level that has a target language translation equivalent at a different level. In other words, it is simply a shift from grammar to lexis.
Category shift refers to departures from formal correspondence in translation. What is meant by formal correspondence is any grammatical category in the target language which can be said to occupy the same position in the system of the target language as the given source language category in the source language system (Machali, 1998: 13). The category shift is divided again into structure shifts, class shifts, unit shift, and intra-system shifts. Structure shift is the changing of words sequence in a sentence. Class shift occurs when the translation equivalent of a source language item is a member of a different class from the original item. Unit shift is the changes of rank; that is, departures from formal correspondence in which the translation equivalent of a unit at one rank in the source language is a unit at a different rank in the target language. Intra-system shift refers to the shifts that occurs internally, within the system; that is for those cases where the source and the target language possess systems which approximately correspond formally as to their constitution, but when translation involves selection of a non-corresponding term in the target language system.
Machali (1998: 152) also proposes the kinds of translation shift. She divides the shift in translation into two kinds: obligatory shift and optional shift. An obligatory shift refers to the kinds of shift that occurs when no formal correspondence occurs in the translation. It is the shift that its occurrence is dictated by the grammar. The other kind of shift is the optional shift. It refers to a case of shift that is caused by the translator's discretion It is called optional shift since the translator could have chosen the more equivalent clauses with the readers’ orientation in the target language text.
In addition, Machali (1998: 160) states that there are two basic sources of translation shifts: source language text-centered shift and target language text-centered shift. The source language text-centered shifts are of three kinds, namely, grammatical shift, which mainly concerns particle markedness, foregrounding, and tenses; shifts related to cohesion, which mainly concern ellipsis; and textual shifts, which mainly concern genetic ambivalence, and embodiment of interpersonal meaning. The target language text-centered shift causes the main problem concerned with achieving effectiveness, pragmatic appropriateness (including the cultural one), and information (referential) explicitness.
Nida and Taber (1969: 171) say that some of the most common shifts in meaning found in the transfer process are modifications which involve specific and generic meaning. Such shifts may go in either direction from generic to specific or specific to generic. A shift may result from a difference of the system in both languages. The difference can be in the form of vocabulary or structure, the shift caused by the vocabulary results in a shift in meaning. It can be concluded that there are two kinds of shifts in meaning. The first is the meaning shift from general to specific meaning. The second is the meaning shift from specific to general meaning. These kinds of shifts often cause incorrect translation. The shift of structure, however, usually does not change the meaning or the message of the original text.