adverb, noun
Only when written 早く
Used for something happening earlier than expected or before a deadline. Also used as a noun in expressions like 早くに.
明日は早く起きなければならない。
I have to get up early tomorrow.
早く帰ってきてね。
Come home soon, okay?
adverb
Used for speed of action or movement. The kanji 速く is especially common for this sense.
彼は速く走れる。
He can run fast.
仕事を速く片付けてください。
Please finish the work quickly.
Adverbial form of the adjective 早い/速い (はやい), from Old Japanese. The two kanji spellings distinguish 'early' (早) from 'fast' (速), though the kana form is common for both.