Well and good can have similar meanings. Well is an adverb, while good is an adjective. The adverb well is used to modify verbs, whereas the adjective good is used to modify nouns.

She writes well. (Here the adverb well modifies the verb writes.)
She is a good writer. (Here the adjective good modifies the noun writer.)

She teaches well. (NOT She teaches good.)
She is a good teacher. (NOT She is a well teacher.)
I like her. She is good. (NOT She is well.)
He speaks English well. (NOT He speaks English good.)
Her English is good. (NOT Her English is well.)
She speaks good English. (NOT She speaks well English.)

Notes

The sentence ‘She speaks well English’ is not correct, because adverbs cannot usually go between a verb and its object.

Well = in good health

There is also an adjective well, meaning ‘in good health’.

‘How are you?’ ‘Quite well, thanks.’
I am not feeling very well.

The adjective well is only used to talk about health.

She is always well when she lives in the mountains. (Here we are talking about her state of health.)
She is always happy when she is with him. (NOT She is always well…)

Though well is an adjective, it is not normally used before a noun. We can, for example, say ‘She is well’, but we do not normally say ‘She is a well girl’.