烂(làn) - rotten, mushy, lousy in Mandarin

烂 (làn) covers a range English splits up: fruit gone rotten, meat stewed until it falls apart, clothes worn through, and anything of miserable quality. The thread running through it is a loss of structure, something that no longer holds together.

Not all of that is bad. 煮烂 and 炖烂 are what you want from a slow-cooked pork belly, and 烂熟 means thoroughly done. So the same word that condemns a peach praises a stew, and the verb in front tells you which reading is meant. When it follows 放 or 坏, as in 放烂了, it is decay.

The third sense is slang and it is blunt. 很烂 about a film, a plan, a restaurant or somebody's work means rubbish, and it stings more than 不好. English speakers who have learnt it as rotten often use it cheerfully about a colleague's report without realising how hard it lands. For a softer verdict, 不太好 or 一般 will do the job.