Samsung
Turn On/Off Auto Spell Check auf Samsung
Auto spell check on the Samsung Keyboard does not autocorrect text. It compares words against a dictionary and underlines anything unrecognized in red, offering a suggestion you accept by typing the next character or word.
By default this option is turned off on many Samsung devices, so typos pass through without any visual warning. Turning it on does not change autocorrection behavior elsewhere in the system, it only affects the red underline and suggestion popup inside the Samsung Keyboard.
To enable or disable it, go to Settings ➔ General management ➔ Samsung Keyboard settings ➔ Auto spell check, then switch the toggle for the keyboard language you use.
Each installed keyboard language has its own toggle. If you type in more than one language, check the setting separately for each one, otherwise spell check may work in English but stay silent in another language.
If you expect full autocorrection instead of underlines, look for a separate 'Auto replace' or 'Predictive text' option in the same Samsung Keyboard settings menu, since that is the feature responsible for automatically swapping in corrected words.