早

早上好 is fine, but it sits one notch formal, like 'Good morning, sir.' Real people in offices, gyms and elevators just say 早 or 早啊. If you only ever filed 早 under 'early', dropping one syllable as a whole greeting feels wrong. Natives hear the missing half of 早上好 and fill it in for you. But 早上 alone never works as a hello, because it only names the hours.

早点 has two lives: breakfast, and 'a little earlier', where 点 does the same softening job as in 快点儿. Put it before the action and you are asking for a favor in advance. Put 早 after the action with 了 and you are reporting a surprise instead. So 你早点来 is a request, and 你来早了 is an observation, often a mild complaint that the food is not ready.

Chinese loves squeezing time into neat pairs, so 早中晚, 昨今明 and 前后左右 all cut the helper syllables. The same shorthand shows up in 今早 and 明早, which are perfectly ordinary in speech. Outside these set phrases, 早 on its own means 'early', never 'morning'. Try naming a morning as a plain noun and it wobbles: 今天的早 is not a thing, 今天早上 is. “早中晚，昨今明。” (Morning, noon, and night; yesterday, today, and tomorrow.) ── [《时间和时候（1）卖车看电影》](/read/time-versus-moment#s1)