adjectives vs. adverbs
Adjectives describe things. Adverbs describe what things do. You have been using the first one for the second job.
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Wrong: She did good on the presentation.
Right: She did well on the presentation.
Good is a thing you do. Well is how you did it. She did good if she volunteered at a shelter afterward.
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Wrong: He sings beautiful.
Right: He sings beautifully.
Beautiful would have to describe him. It’s describing the singing.
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Wrong: That went real quick.
Right: That went really quickly.
Two in four words. Real was modifying an adjective, and quick was modifying a verb.
The test
Ask what the word is describing: the thing, or the doing.
Describing the thing is an adjective—a quiet room, a careful driver. Describing the doing is an adverb—speaks quietly, drives carefully. Same word, different job, different form.
He sings beautiful fails the test instantly, because beautiful has nothing to attach to except him, and that is not what you meant.
Before you argue
You are about to say drive safe, and you are not entirely wrong. Three real exceptions, and none of them is a general licence.
Flat adverbs exist. A handful of words have worked as both since long before anyone was mad about it: drive fast, work hard, go slow, sit tight, hold still, aim straight. Adding -ly to those sounds worse, and in a few cases means something else—hardly working is not working hard. This list is closed. You cannot extend it to speak clear because you like how it sounds.
Imperatives get a pass out loud. Drive safe and sleep tight are fossilized as spoken goodbyes and nobody will blink. Put drive safe in a written sentence with a subject and a tense, and it stops being a goodbye.
Think different was a slogan. It works because it copies the shape of think big, where the word is the thing being thought rather than a description of the thinking. It was written that way on purpose by people who were paid well to make you notice. You are not doing that in a status update.
The other direction is worse
I feel badly is the hypercorrection, and it means your fingers are numb.
After be, feel, seem, look, smell, sound, and taste, the word points back at the subject, not at the verb—so it takes the adjective. I feel bad. It looks great. That smells terrible. She seems happy. Nobody says the milk smells badly, because the milk is not the one doing the smelling, and I feel badly is the same sentence with the same problem.
Reaching for -ly to sound more careful is how you end up sounding less careful. Pick the word that does the job.
Drive safe. Write safely.