These two adjectives express similar meanings and are read the same way, but use different kanji. Let’s capture an image of what each kanji means so we can use them properly!
Translation: early; soon
It expresses that something is happening or done earlier than some standard or expected times.
“I hope it will be spring very soon.”
*Using the calendar date as a basis, the speaker wants spring to come as soon as possible.
“Please come as soon as possible.”
*Using the time on the clock as a basis, the speaker wants someone to come as soon as possible, even if it’s a minute or a second.
”Please let me know as soon as you get the results of the (medical) tests.”
*Using the usual date for receiving test results (next clinic appointment) as a basis, the speaker wants them to move up the date and inform the results as soon as possible, even by phone.
Translation: fast; quick; rapid
It expresses that something is happening quickly, taking a short amount of time or how fast things are done.
“Which medicine for headaches works fast?”
“Their conversation speed is too fast for me to keep up with them.”
The antonym for both kanji is「遅い」(おそい).
↔antonym for 早い
“I woke up later than usual today.”
antonym for 速い
“My sister walks slowly.”
“The bus arrives earlier.”
It expresses that the bus will arrive earlier than other transportation methods, including trains that are faster than buses.
“Buses are faster (than certain vehicles).”
It expresses that buses are faster than other vehicles (e.g., bicycles and electric scooters).
Trains that run early in the morning (or come before the train being compared)
Fast trains like express trains with a short time to arrive
“You’re early today.”
The speaker talked to a colleague who arrived at work earlier than his usual (standard) arrival time, which was 8:30 am.
“You must have prepared fast and rushed to the office.”
The speaker talked to a colleague who said he had overslept on the phone but arrived at work within 10 minutes after the call.
In the following idioms, the kanji「早い」is used even though the meanings are closer to 「速い」.
“to rot quickly”
*「足が速い」simply indicates a fast running speed, but when this kanji is used, it indicates that food easily rots.
“to recover quickly from illness”
“to catch on first”; “to be a quick learner”
“to make a long story short”