Diaz-Cintas (Diaz-Cintas, 1999) articulated in his paper “Subtitling: Diagonal Translation”, time, space, music, image, phonetics, and spoken language are six types that restricting subtitle translation. Among the six factors, time and space have more effect on translation. Subtitles should convey information in the form of lines at the bottom of screen within a certain period of time, which means that language should cooperate simultaneously with image. Space limitation results from the difference between Chinese and English—English occupies more space than Chinese. Gottlieb also regards subtitle translation as diagonal, transformation between oral signals and written words.

3    Translation Theories
3.1 Functional Equivalence Theory
In the field of functional translation theory, American linguist and translator Eugene Nida’s efforts are the most influential. In 1964, Nida published his book “Toward a Science of Translating”. In his book, Nida put forward his theories: “translation as science”, “equivalence theory” and “communicative translation”. He emphasizes a natural equivalence of the meaning between source language and target language. The translated version should be as natural as if written by a native speaker. In his point, there are two kinds of equivalence: formal equivalence and dynamic equivalence. Formal equivalence focuses on the structure and content of the message itself. It is a source language-oriented, aiming at retaining the form and content of source language. Dynamic equivalence pays more attention to a dynamic relationship—the relationship between target language and target reader should be equivalent with that between source language and source text readers. In 1986, Nida presented the conception of functional equivalence. He proposed four primary requirements in regard to functional equivalence translation principles: first, translation should be meaningful; second, it should convey both meaning and form; third, the expression of translation should be natural and fluent; fourth, target readers should have the same response as source language readers.

3.2 Skopos Theory
Skopos is a Greek word for “purpose”. Different from Nida’s functional equivalence theory, which is source language-oriented, Skopos theory attaches more importance to the “target”. According to Skopos theory, the primary principle determining any translation process is the purpose of the overall translation action. Usually it refers to communicative purpose in the target text. Han J. Vermeer developed “Skopos theory” in 1978. According to Vermeer, translation is and intentional and purposeful action of human, not just a code-switching process. In Vermeer’s point of view, each text is provided for a given purpose and should serve this purpose. One of the translation rules of Skopos theory is the skopos rule which reads as follows: translate/ interpret/ speak/ write in a way that enables your text/ translation to function in the situation in which it is used and with the people who want to use it and precisely in the way they want it to function. (Nord, 2001:29). There are two other rules of this theory, namely coherence rule and fidelity rule, which means that a translation should be acceptable in a sense that is coherent with the receivers’ situation and that the target text should be inter-textually coherent with the source text. Fidelity rule is subordinate to coherence rule while both are subordinate to skopos rule. “生活大爆炸”中美式幽默的翻译方法研究(4):http://www.youerw.com/yingyu/lunwen_7855.html