真心(zhēn xīn) - sincere, heartfelt; sincerely in Mandarin

真心 (zhēn xīn) is sincerity, the genuine article as opposed to politeness or performance. It runs in three grammatical directions: as an adverb before a verb, 真心喜欢, genuinely like; as a predicate adjective with 的, 他是真心的, he means it; and as a noun in the set phrase 一片真心, a whole heart's worth of sincerity.

In real speech the adverbial use dominates. Young speakers online have pushed 真心 further still, so 真心累 means genuinely knackered rather than sincerely tired, which is closer to the English intensifier really than to sincerely.

The trap is the opposite word. English speakers reach for 假心 by symmetry, and it does not exist; the antonym is 假意 or simply 不是真心的. Note too that 真心话 means a truthful answer, as in the game truth or dare, and not a sincere speech.