The word すごい/Sugoi and すごく/Sugoku both sound similar and they are both frequently used words. すごい(Sugoi) is usually translated as “Amazing”, “Terrific”, “Awesome” and etc. すごく(Sugoku) is translated as “Very” or “Awfully”.
But can you tell the difference between them? I think it’s difficult to explain which one to use when and why. When looked up in the dictionary, すごい(Sugoi) is an adjective and すごく(Sugoku) is an adverb that has been transformed from the conjugated form of すごい(Sugoi).
すごく(Sugoku) can be used when it’s placed before the declinable words which are verb, adjective and adjectival noun. In other words, you don’t use すごく(Sugoku) when followed by nouns. Therefore, the sentence that has すごく(Sugoku) + Noun is wrong and it should be すごい(Sugoi) + Noun. Because すごい(Sugoi) is an adjective and adjective usually modifies the noun that comes after.
Here is the diagram of the basic rule.
However, some dictionaries explain as if すごい(Sugoi) can be used instead of すごく(Sugoku) and can be added to declinable words. It says such usage is “considered non-standard” and does not say it’s necessarily wrong. But you should use すごい(Sugoi) + declinable words only in casual conversation. And in official documents and in business situations, you should stick to the rules of original proper usage of them.
How to avoid misuse of the words すごい(Sugoi) and すごく(Sugoku)?
It is important to remember that すごく(Sugoku) is a conjugated form and adverb, so noun canNOT come after it. Therefore, phrases such as “すごく人(ひと)/すごくひと/Sugoku Hito” is wrong and does not make sense because, in English, this phrase means “Very person”. Instead, this should be “すごい人/すごいひと/Sugoi hito/Amazing person”.
The problem is the usage of “すごい(Sugoi)”. In modern Japanese, people often use すごい(Sugoi) before the declinable words.
すごいかわいい/Sugoi Kawaii/Very cute
すごい大きい(おおきい)/Sugoi Ookii/Very big
Those usages can be seen even on TVs and newspapers, but it’s not the proper usage and should not be used in formal situations. Some Japanese don't know about proper usage, but many people feel unnatural. As a basic rule, it is easy to understand if you remember that すごい(Sugoi) is followed by a noun and that すごい(Sugoi) is not used when no noun follows.