a sentence group: a supporting sentence, then the main one, linked across the full stop
Structure
偏（原因／条件／让步…） 。 因此／否则／尽管如此／为此… ， 正（主要的意思） 。
What it means
A sentence group where one sentence leans on another: a supporting 偏 part (cause, condition, purpose, contrast, supposition, concession) and a 正 part with the main point, usually last.
How it is built
偏 sentence。+ linking word + ， + 正 sentence。: 孩子在信里只说好事。尽管如此，父母还是读出了他的辛苦 (the child wrote only good news. Even so, his parents read his hardships).
When to use it
Across a full stop, open the next sentence with a word that points back: 因此 (cause), 只有这样 (condition), 为此 (purpose), 然而 (contrast), 否则 (supposition), 尽管如此 (concession). Inside one sentence use pairs: 因为…所以, 虽然…但是.
Watch out
Do not cut off a 因为 or 虽然 clause as its own sentence: ✗他没告诉她。因为怕她伤心。 ✓他没告诉她，因为怕她伤心, or open the second sentence with 这是因为.
Examples
长安被叛军占领，杜甫被困城中，和家人断了音信。因此，他写下了“家书抵万金”。
Chang’an was in rebel hands and Du Fu was cut off from his family. And so he wrote “a letter from home is worth ten thousand in gold”. (cause, then result)
孩子在信里只说好事。尽管如此，父母还是读出了他的辛苦。
The child wrote only good news. Even so, his parents read his hardships between the lines. (concession, then the main point)
到了学校马上给家里打电话。否则，妈妈又要一夜睡不着了。
Ring home the moment you get to school. Otherwise Mum will lie awake all night again. (supposition: here the main point comes first, and 否则 says what happens if not)
Learn this in Lesson 226 · HSK 9+: 家书 (Letters home)
A dialogue, a reading, word cards with audio and a drill that uses it, in Learn Chinese with Bao. The first lessons of every level are free.