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When you're typing through, the system checks your spelling and suggests a few words. Unless you've selected one of them, the highlighted suggested word overwrites your word when you finish typing it followed by a space, punctuation mark or return character. If you feel that's annoying, you can turn off Auto-Correction.
Open the app Settings and select 'Language & input' under PERSONAL.
Tap the Quick Settings icon next to 'Android keyboard'.
Select 'Auto correction' under TEXT CORRECTION.
Select 'Off' to turn off 'Auto-Correction'.
Note: As an alternative, whenever a word is replaced not to your liking, you can tap the Delete key immediately after which and the system returns the word you've typed.