Auto-correct can be a source of many misunderstood texts, so let's check that out. One option to delete words one-by-one would be to press and hold on them as they show up to remove suggestions individually. If you wanted to start with a clean slate, go to Settings > Language and Input, then tap on "On-screen Keyboard" and look for "Samsung Keyboard." Scroll down to Reset Settings > Clear Personalized Data to delete everything you've entered through normal use.
While inside the Samsung Keyboard section, tap on Predictive Text > Text Shortcuts to add suggestions of your own. You'd see an example of this within that menu. Let us know if that helps with what you needed.
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