Language will vary depending on who is using it and what dialectsformalitynon-standardregistersettingsociolectsspecificstrangertopicvocabulary it is used in.
Special types of English (football language, talking about hobbies, medical language and so on) will demand special dialectsformalitynon-standardregistersettingsociolectsspecificstrangertopicvocabulary and, often, special structures. This is a question of dialectsformalitynon-standardregistersettingsociolectsspecificstrangertopicvocabulary.
Style refers to the level of dialectsformalitynon-standardregistersettingsociolectsspecificstrangertopicvocabulary so we would say I wonder if you can help me? when talking to a dialectsformalitynon-standardregistersettingsociolectsspecificstrangertopicvocabulary but might just say Help! when calling to a friend.
People vary, too, and so will their language.
People from particular areas (such as Southern Britain or Canada) will have a national or regional dialectsformalitynon-standardregistersettingsociolectsspecificstrangertopicvocabulary and people from certain classes will also have different ways of speaking or dialectsformalitynon-standardregistersettingsociolectsspecificstrangertopicvocabulary.
We should be careful about teaching regionally dialectsformalitynon-standardregistersettingsociolectsspecificstrangertopicvocabulary language.
We should know what our learners need English for so occupation and dialectsformalitynon-standardregistersettingsociolectsspecificstrangertopicvocabulary are important here.
We should be careful if a song or other text contains instances of dialectsformalitynon-standardregistersettingsociolectsspecificstrangertopicvocabulary or regionally-influenced language.