One of the common grammatical problems is confusing adjectives and adverbs.
What is the difference between adjectives and adverbs?
We use adjectives with nouns to speak about features and qualities and answer the questions what, what kind of: a nice person, a personal computer, a black T-shirt.
Adverbs are used with verbs and answer the questions how, when, where, or to what extent: she sings beautifully, they reacted immediately, I was extremely excited.
good (adjective) — well (adverb)
He is a good worker. What kind of worker is he?
He works well. How does he work?
heavy (adjective) — heavily (adverb)
We arrived late because of the heavy traffic. What was the traffic like?
It has been raining heavily all day. How has it been raining?
thorough (adjective) — thoroughly (adverb)
The products were packaged after a thorough check. After what kind of check?
Add some water to the powder and mix thoroughly. How should I mix them?
How to make adverbs
We normally make adverbs out of adjectives by adding -ly at the end of an adjective.
But there are plenty of exceptions.
Sometimes the adjective and the adverb have the same form: fast, far, cool etc. Sometimes the word ending with -ly is not an adverb but an adjective: lovely, friendly, lonely, silly etc. And sometimes the meaning is completely different: hard — hardly, late — lately.
It's better to look the word up in a dictionary every time you are not sure.
We use adjectives but not adverbs after some verbs — be, sound, feel, look etc.
The weather is nice today. WRONG The weather is nicely.
Why do you feel sad? WRONG Why do you feel sadly?
This track sounds good. WRONG This composition sounds well.