复(fù) - again, to repeat, to reply in Mandarin

复 (fù) carries a bundle of related ideas: again, back, repeated, doubled, and by extension to reply. It is almost always a bound morpheme in modern Mandarin, so you meet it inside compounds rather than standing on its own.

The high-value ones are worth learning as a set. 复习 is to revise, 复印 to photocopy, 重复 to repeat, 恢复 to recover or restore, 复杂 complicated, 答复 and 回复 to reply. Once you can see 复 as "again or doubled", most of that list explains itself.

The trap is expecting to use it alone. A sentence like 我复了 is not Chinese; you need 我回复了 or 我答复了. The one place a bare 复 survives is in set literary phrases such as 一去不复返, where it works as an adverb meaning "again". Note too that the simplified form has swallowed two different traditional characters, 復 and 複, so a traditional text will spell 复习 and 复杂 with different first characters.