I've seen "today" translated in several ways: "今日", "今天", "本日".
Are they all official words? What's the difference among them? Are those words more or less interchangeable in common language? Are there any other expressions? Are there so many ways to indicate "yesterday" and "tomorrow"? Can you provide examples?
A similar question (and useful answers) have already been provided at "Today" in Spoken Singaporean Mandarin.
Thanks!
In mainland China, we say 今天 mostly. I have never heard of 本日 personally. 今日 should only be used in a formal setting like a newspaper or a TV program.
今日: Tends to be poetically used rather than day-to-day. It is still used, but usually in literature such as poetry, set phrases rather than on its own (like 今日晚间), or written documents and names (such as 今日新闻). It's used, but it's really not the most common choice of word for people on the mainland.
今天: The most common; used in regular speech in modern Mandarin.
本日: I've only ever seen this used in Japanese, actually. 本日 is pronounced ほんじつ (honjitsu) in Japanese, and is a formal word for "today", but it's never used in casual speech, only formal occasions such as in business. After a google search, it seems to be used in Chinese sparingly, and matches the pattern of 本年，本月，本日, but I've never seen it used in Chinese before today, only Japanese.
So, in reality I'd say there are only 2 that are actually used, and even of these, 今天 is the only one you really need to be using unless you have a reason not to.
"今天" we use this most often, and when you put it with "晚上" you can say "今晚" which means this evening. "今日" is more used in written Chinese and you can see this in newspaper, Tv programs. For example many TV channels in China have programs like "今日新闻"“今日热点”. As to "本日", I rarely hear this. But we use “本” on formal occasions sometimes, for example “本公司” “本人”"本店" which refers to "our company""myself"“our store”. Hope this can be of some help.
3 is not many! Can only think of 2 for yesterday and tomorrow! Maybe you know these songs.
昨天/昨儿个，我的所有困难好像都不在了。
明天/明儿个 你是否仍将爱我？
今朝 is another way to say today. Nowadays it's only used in local dialects. But this poem is very famous: 今朝有酒今朝醉，明日愁来明日愁.
Basically, 本(more formal)、今 has the same meaning - this, and 日(more formal)、天 has the same meaning - day, and the first one is more formal.
You can put them together in the three ways you mentioned, but I never heard anyone use 本天 because it's probably a combination of formal and informal, so it sounds weird.
