彼此 / 如此: each other, so, such
【五01】指示代词 / pronouns
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
彼此 and 如此 are two formal demonstrative pronouns. 彼此 means “each other, both sides”: 朋友之间应该彼此信任 (friends should trust each other). 如此 means “so, in this way, such”: 如此认真 (so serious).
彼此 works like 互相 (hsk3-g76) before a verb, but it can also stand alone, almost like a noun: 不分彼此 (not to tell “yours” from “mine”), 彼此彼此 (“same to you”, the modest answer to a compliment). Do not stack 彼此 and 互相.
如此 replaces 这样 or 这么 in more careful speech and writing. It already gives the degree: 如此重要, never 如此很重要. It can also pick up a whole idea: 事情就是如此 (that is how things are), 尤其如此 (this is especially true), 并非如此 (this is not the case).
Examples
朋友之间应该彼此信任。
Péngyou zhījiān yīnggāi bǐcǐ xìnrèn.Friends should trust each other.
我们认识十年了，彼此都很了解。
Wǒmen rènshi shí nián le, bǐcǐ dōu hěn liǎojiě.We have known each other for ten years and understand one another well.
“你的汉语说得真好！”“彼此彼此。”
“Nǐ de Hànyǔ shuō de zhēn hǎo!” “Bǐcǐ bǐcǐ.”“Your Chinese is really good!” “Likewise.”
这件事如此重要，我们必须认真对待。
Zhè jiàn shì rúcǐ zhòngyào, wǒmen bìxū rènzhēn duìdài.This matter is so important that we must take it seriously.
他们是多年的好朋友，从来不分彼此。
Tāmen shì duōnián de hǎo péngyou, cónglái bù fēn bǐcǐ.They are old friends who have never drawn a line between “yours” and “mine”.
我以为他会生气，可事实并非如此。
Wǒ yǐwéi tā huì shēngqì, kě shìshí bìngfēi rúcǐ.I thought he would get angry, but that was not the case.
Common mistakes
✗ 他们彼此互相帮助 → ✓ 他们彼此帮助 / 他们互相帮助 (do not stack the two, they say the same thing).
✗ 他如此很努力 → ✓ 他如此努力 (如此 already gives the degree, so no 很).