I am not a lawyer. I am thankful for that. However living in Hong Kong inevitably exposes you to legal issues because what passes for political discourse in this city revolves around a seemingly simple piece of law. The Basic Law is, in theory, Hong Kong's constitution. It was negotiated between the British and Chinese prior to the handover in 1997 as a way of assuaging British guilt at not introducing democracy to the place in the couple of hundred years they ran it. Like most such documents there is a mix of the bland and ambigious, designed to make it an uninteresting as possible. However it has become an important document to base the aspirations of Hong Kong's democracy movement.

For example, Article 45 in full reads

The Chief Executive of the Hong Kong Special Administrative Region shall be selected by election or through consultations held locally and be appointed by the Central People's Government.

The method for selecting the Chief Executive shall be specified in the light of the actual situation in the Hong Kong Special Administrative Region and in accordance with the principle of gradual and orderly progress. The ultimate aim is the selection of the Chief Executive by universal suffrage upon nomination by a broadly representative nominating committee in accordance with democratic procedures.

The specific method for selecting the Chief Executive is prescribed in Annex I "Method for the Selection of the Chief Executive of the Hong Kong Special Administrative Region". (my emphasis)

If there is a need to amend the method for selecting the Chief Executives for the terms subsequent to the year 2007, such amendments must be made with the endorsement of a two-thirds majority of all the members of the Legislative Council and the consent of the Chief Executive, and they shall be reported to the Standing Committee of the National People's Congress for approval.

...

With regard to the method for forming the Legislative Council of the Hong Kong Special Administrative Region and its procedures for voting on bills and motions after 2007, if there is a need to amend the provisions of this Annex, such amendments must be made with the endorsement of a two-thirds majority of all the members of the Council and the consent of the Chief Executive, and they shall be reported to the Standing Committee of the National People's Congress for the record.

Problem is China has realised they don't want Hong Kong to be too democratic. It would look like the Central Government is giving in to the voice of the people (500,000 marchers back in July and smaller protests since). It would give the people a voice. It would be the thin edge of the wedge. So the PRC does what anyone in their situation would: they slam the door. From the SCMP

After a week of dominating Hong Kong headlines, two leading mainland legal experts on the Basic Law return to Beijing today having delivered a clear message: go slow on constitutional change. They told the city in no uncertain terms it cannot elect the chief executive by universal suffrage by 2007. Think about 2030 or 2040 instead, they said.

While former Basic Law drafter Xiao Weiyun and jurist Xia Yong say they do not represent the central government, their view that Hong Kong can only contemplate introducing universal suffrage after 2030 is believed to reflect official thinking. Analysts say their visit has been arranged to informally articulate the central government's position ahead of a visit to Beijing by a Hong Kong government taskforce on political reform, and to dampen aspirations for more rapid democratisation.

...

The mainland experts reiterated Beijing had a decisive say in Hong Kong's political development. At a public forum yesterday, Professor Xiao said the Basic Law drafters never considered election of the chief executive by universal suffrage in 2007. Referring to Article 45 of the Basic Law, which says the chief executive will ultimately be elected by universal suffrage, Professor Xiao said it meant this would be achieved in the final stage of the 50-year lifespan of "one country, two systems". But, he said, "it did not necessarily mean that it would not be achieved until the 2030s or 2040s, or [that] it was not allowed in the 2020s".

The Basic Law says the methods for electing the chief executive and Legislative Council should evolve in light of the "actual situation" in Hong Kong, and in accordance with the principle of "gradual and orderly progress".

The ultimate aim is the selection of the Chief Executive by universal suffrage. Ultimate being within the next 4000 years or so.