How to turn off (grammar and spellchecking) auto-correct?

You can turn off spelling and grammar corrections on Mac by clicking on "Edit" in the menubar, then "Spelling & Grammar" and uncheck:

• "Check Spelling While Typing",
• "Check Grammar With Spelling",
• "Correct Spelling Automatically".

You can also add new words or ignore words from the spellcheck under "Show Spelling and Grammar."