You cannot demonstrate feelings (hugging, kissing) in public places. This is considered rude. Patience, punctuality, and deference are the characteristic of a good upbringing. Hands should be visible when talking. If you keep them in your pockets or behind your back, cross them, then the locals may perceive this as rude. Personal questions are considered the norm here. Local residents will ask you about your family and your marital status. Shanghai residents are sympathetic to people who don't have a family. Chinese are quite straightforward in their statements. Therefore, you should not be offended if someone says something unflattering about your appearance. This is not an attempt to offend but it's accepted here.
You should be very careful when talking about politics. There are two sensitive subjects here. These are Taiwan and Tibet. The ambiguity of the status of both territories is very unpleasant for the Chinese. Therefore, it is better to avoid these topics. You should not talk about customs and traditions. It is not acceptable to refuse directly. Most likely, a local resident will try to answer ambiguously or change the topic. Direct refusal is perceived as rudeness. Special rules apply to gifts. For example, you cannot gift a watch. This is a symbol of death and parting here. You can't make green gifts. You can present a small souvenir as a sign of respect. It is customary to open a gift immediately, in front of the giver.
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