Japanese has many words that sound the same but have different meanings.
One example is hayai.
You might say, 「今日(きょう)ははやい時間(じかん)に起(お)きたよ。」,
or on the train you might say, 「この電車(でんしゃ)ははやいね。」 .
Both are read as hayai, but the kanji is different.
Explanation
Both 「早い」 and 「速い」 are i-adjectives. They are read the same way, but they are used differently depending on whether you are talking about time or speed. In English, this is similar to the difference between "early" and "fast". Both words are connected to time, but 早い focuses on being before the expected time, while 速い focuses on speed.
早い (hayai)
早い means something happens earlier in time or order. It focuses on time and sequence.
Examples:
朝(あさ)が早(はや)い。 (The morning is early.)
会議(かいぎ)が早(はや)く終(お)わる。 (The meeting ends early.)
電車(でんしゃ)が早(はや)い時間(じかん)に出(で)る。 (The train leaves at an early time.)
子(こ)どもは成長(せいちょう)が早(はや)い。 (Children grow up early.)
速い (hayai)
速い means something has speed. It describes how quickly someone or something moves or progresses.
Examples:
足(あし)が速(はや)い。 (He runs fast.)
車(くるま)が速(はや)い。 (The car is fast.)
反応(はんのう)が速(はや)い。 (The reaction is quick.)
インターネットが速(はや)い。 (The internet is fast.)
Common Situations
Even though both are pronounced hayai, the choice of kanji changes the meaning.
電車(でんしゃ)が早(はや)く出(で)た。 (The train left earlier than scheduled.)
電車(でんしゃ)が速(はや)く走(はし)る。 (The train runs fast.)
彼(かれ)は朝(あさ)が早(はや)い。 (He wakes up early in the morning.)
彼(かれ)は足(あし)が速(はや)い。 (He runs fast.)
As Adverbs: 「早く」 and 「速く」
The adjectives 「早い」 and 「速い」 can also become adverbs: 「早く」 and 「速く」.
The distinction remains the same.
早(はや)く起(お)きる。 (To wake up early.)
早(はや)く始(はじ)まる。 (To begin earlier than expected.)
速(はや)く走(はし)る。 (To run quickly.)
速(はや)く読(よ)む。 (To read quickly.)
When using them as adverbs, think the same way: 早く relates to time, 速く relates to speed.
Final thoughts
早い / 早く: Refers to time being earlier or ahead of schedule.
速い / 速く: Refers to movement or speed being fast.
Same pronunciation, but the meaning is clearly different.
In spoken Japanese, people often understand you even if you mix them up. But when you write, choosing the right kanji makes your Japanese more natural and accurate.