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but I meet sentences, where adjectives are used instead of adverbs, so I don't understand why?

Some words may be either an adjective or an adverb, depending on the usage:

Ex: "just"

1. "I saw just you."

Here, the word "just" is an adjective, modifying the pronoun “you”. It means “You are THE ONLY ONE I saw – I saw no one else.”

2. "I just saw you."

Here, the word "just" is an adverb, modifying the verb “saw”. It tells more about WHEN I saw you: “I saw you VERY RECENTLY.”

However, you will often see adjectives MISUSED as adverbs in English. It is done so often in some cases that people forget it is an error.

Example 1:

Person A: "How's your mother?"
Person B: "She's doing good."

This is using the adjective "good" in place of the adverb "well". This sentence ("She's doing good.") actually winds up making "good" a noun, and the meaning becomes, "She is doing good deeds." The correct English is "She is well."

Example 2:

"I read fast!"

Here, the adjective "fast" is incorrectly used as an adverb. Correct English is "I read rapidly" or "I read quickly."

And, likewise, people use adverbs where they should use an adjective:

Person A: "My dog died today."
Person B: "Aww ... I feel badly."

Here, the adverb "badly" was incorrectly used in place of the adjective "bad". The actual meaning of this sentence ("I feel badly") would be "My sense of touch is not good"!

Just because you see something in writing does NOT mean it is correct English. This forum seems like a great place to ask, if you have specific questions as to the use of an adjective or adverb in a sentence you encounter.