suǒ yǐ
我和他在一起工作过，所以对他比较熟悉。b）用在上半句主语和谓语之间，提出需要说明原因的事情，下半句说明原因：我所以对他比较熟悉，是因为我和他在一起工作过。也说之所以。c）上半句先说明原因，下半句用“是……所以……的原因（缘故）”：我和他在一起工作过，这就是我所以对他比较熟悉的原因。d）“所以”单独成句，表示“原因就在这里”：所以呀，要不然我怎么这么说呢！
Wǒ hé tā zài yìqǐ gōngzuò guò, suǒyǐ duì tā bǐjiào shúxī. b) Yòng zài shàng bàn jù zhǔyǔ hé wèiyǔ zhījiān, tíchū xūyào shuōmíng yuányīn de shìqing, xià bàn jù shuōmíng yuányīn: Wǒ suǒyǐ duì tā bǐjiào shúxī, shì yīnwèi wǒ hé tā zài yìqǐ gōngzuò guò. Yě shuō zhī suǒyǐ. c) Shàng bàn jù xiān shuōmíng yuányīn, xià bàn jù yòng 'shì……suǒyǐ……de yuányīn (yuángù)': Wǒ hé tā zài yìqǐ gōngzuò guò, zhè jiù shì wǒ suǒyǐ duì tā bǐjiào shúxī de yuányīn. d) 'Suǒyǐ' dāndú chéng jù, biǎoshì 'yuányīn jiù zài zhèlǐ': Suǒyǐ ya, yàobùrán wǒ zěnme zhème shuō ne!
I worked with him before, so I'm quite familiar with him. b) Used between the subject and predicate in the first half of a sentence to present something that needs explanation, with the second half explaining the reason: The reason I'm quite familiar with him is because I worked with him before. Also expressed as 'zhī suǒyǐ'. c) The first half explains the reason first, the second half uses 'shì...suǒyǐ...de yuányīn (yuángù)': I worked with him before, this is the reason why I'm quite familiar with him. d) 'Suǒyǐ' as an independent sentence, expressing 'the reason is right here': That's why! Otherwise, why would I say this!
忘其所以
wàng qí suǒyǐ
to forget oneself; to be beside oneself (with joy, excitement, etc.); to lose one's composure
不知所以。
bù zhī suǒyǐ
not to know the reason; to be baffled; to not understand why