早い and 速い: early timing or high speed
Both words are read はやい, but 早い concerns early timing or a short delay, while 速い concerns high speed. Use context to interpret 遅い and the adverb はやく as well.
早い: early or prompt
早い applies when something happens earlier than a schedule or ordinary reference point. 今日は朝早く起きました。 and 予定より早く着きました。 describe waking and arriving at an early time.
返事が早い uses 早い for a prompt reply with little delay. It evaluates how soon the event occurs, rather than describing an object moving at high speed.
速い: high speed or a fast rate
速い applies when a person, vehicle, or action proceeds at high speed. 新幹線は速いです。 and あの選手は走るのが速いです。 compare the speed of movement with another object or standard.
読むのが速い describes a fast reading rate. It does not mean that the person started reading early, which would involve 早い. Ask whether the issue is when an event occurred or how quickly it progressed.
Use context for 遅い and はやく
遅い can oppose either meaning. 到着が遅い means that the arrival is late, while 走るのが遅い means that someone runs slowly. Identify the comparison instead of pairing one spelling with every use of 遅い.
Both adjectives replace い with く to form the adverbs 早く and 速く. 早く来てください。 asks someone to come early or soon, while もっと速く走ってください。 asks for a higher running speed. When only はやく is written, use the context to decide.
Quick check: Which spelling fits “this train is convenient because it travels fast,” 早い or 速い?
Show the answer and why
Use 速い, as in この電車は速くて便利です。 The sentence refers to high travel speed, not to the train arriving at an early time.