It depends on the type of speech. In an informal speech, slang is fine; in a formal speech, you should avoid slang.
informal
The type of speech depends on where the character is from or their background. How do they speak? If you have a character that comes from the south then you would use slang like 'y'all' or 'howdy' etc.
Not that I can see. He did use a lot of idioms and metaphors, though.
You would be more likely to use slang in casual speech when writing dialogue. Slang is informal language used in casual settings to create a conversational and authentic tone among characters. It is less suitable for formal dialogue where more proper and polished language is expected.
People usually curse to express strong emotion, mostly anger and frustration with a person, an object, or a circumstance. People use slang usually in order to fit in with a particular group. Slang is usually used in casual or humorous speech. It would not be appropriate to use slang in formal contexts such as in the business or professional world, at school, or in religious settings such as in church.
Americans love speech. Since we founded the country, we have loved speech-making and the sound of words. Americans just like using idioms and slang to make speech more colorful and interesting.
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Colloquial language.
You should avoid using slang in formal writing, professional environments, academic settings, and when communicating with individuals who may not understand the slang terms being used. It's important to choose language that is clear, respectful, and appropriate for the context.
Informal speech is any use language that would not be acceptable in formal speech or writing (including both colloquialisms and slang). Colloquialisms are informal speech (particularly words and phrases) that are characteristic of conversation, and may also be typical of sub-dialects. Slang is informal speech (words and phrases) that would be considered undignified in certain contexts, or the use of shared synonyms and metaphors to avoid the taboo on certain topics.
Casual conversations with friends or in informal settings, such as chatting, texting, or social media.
It is slang in only the spoken word. It cannot be used in proper English. As slang, and has to be translated into English before it can be categorized: it means "what will . . . ".What is an interrogative pronoun.Will is part of the future tense of a verb, as in will go, will find.You should always use "What will" in school, not the slang.