之所以…是因为…: the reason why… is that…
复句 / complex sentences
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
In an ordinary causal sentence (因为…所以…) the cause comes first. 之所以…是因为… flips the order: you start from the known result and put the spotlight on the cause, "The reason why… is that…".
之所以 goes after the subject; the second part starts with 是因为 (or 是由于、在于 in writing). A staple of arguments, editorials and HSK essays.
Examples
这家公司之所以成功，是因为它非常重视客户的意见。
Zhè jiā gōngsī zhīsuǒyǐ chénggōng, shì yīnwèi tā fēicháng zhòngshì kèhù de yìjiàn.The reason this company succeeded is that it really values customer feedback.
我之所以选择这个专业，是因为对经济感兴趣。
Wǒ zhīsuǒyǐ xuǎnzé zhège zhuānyè, shì yīnwèi duì jīngjì gǎn xìngqù.I chose this major because I'm interested in economics.
房价之所以这么高，主要是由于供不应求。
Fángjià zhīsuǒyǐ zhème gāo, zhǔyào shì yóuyú gōng bú yìng qiú.House prices are so high mainly because demand outstrips supply.
Common mistakes
✗ 之所以他成功… → ✓ 他之所以成功… (之所以 after the subject).
✗ 之所以…所以… → the second part must be ✓ 是因为….
✗ 因为…之所以…: can't flip it back.