What cannot be registered as trademark?
A trademark has to have one basic feature that is it should be unique and create a brand identity for a product. So if a trademark is such that does not create any brand for a product can't be trademarked. A trademark should not be a conflicting trademarks with others. It means the trademarks should not create confusion between two trademarks. In general,
-Generic words can't be trademarked. For example you can't trademark the words like TV, Fridge, scooter, car etc.
-The names of the cities and countries can not be trademarked.
-The names of Gods and Godesses and the names of religious books can not be trademarked. For example you can't trademark *Lord Ram* or *Ramayana*
-Surnames can not be trademarked under normal circumstances. For Example you can't trademark *Sharma*
-Names of Constitutional Posts or Government posts can't be trademarked. For examples you can't trademark *Prime Minister of Vietnam
-Words which denote illegal or Immoral acts can't be trademarked. For example you can't trademark *Let's cheat* or *Let's grope*
-Words which are prohibited under names and emblems act can't be trademarked. For example, you can't trademark the official sign of Government of Vietnam.
Hope this helps!
Source: Quora