groan、moan、sigh与sob这些名词（动词）均有“叹息声、叹息”之意。
groan : 侧重指因受到难忍受的精神负担或强烈的痛苦等而发出沉重的呻吟，也指显示强烈不满的叹息声。
We could hear the groans of the wounded soldiers.
我们能听到伤兵们的呻吟声。
He looked at the piles of dirty dishes and gave a groan of dismay.
他看着成堆的脏盘子，发出一声沮丧的叹息。
to groan with pain/pleasure
痛苦地呻吟；高兴得直哼哼
He lay on the floor groaning.
他躺在地板上呻吟。
moan语气比groan强，多指拉长的或连续的悲叹（声）或呻吟（声）。
He moaned with pain before losing consciousness.
他痛苦地呻吟，随后就失去了知觉。
"Let me die," he moaned.
“让我死吧。”他呻吟道。
Thelma's always moaning (about something), and forgets how lucky she actually is.
特尔玛总爱抱怨，忘记了实际上她是多么地幸运。
sigh : 多指无意识地发出表示沮丧、悲痛、悔恨或厌倦等情绪的深沉叹息。
She sighed deeply and sat down.
她深叹一口气，坐了下来。
"I wish he was here," she sighed (= she said with a sigh).
“我希望他在这里，”她叹息道。
He leaned back in his seat with a sigh.
他向后靠在座位上长出了一口气。
sob : 指抽噎声或呜咽声，或边说边哭。
You're not going to help matters by lying there sobbing!
你躺在那里哭并不能解决问题！
I could hear her sobs from the next room.
我听见隔壁传来她的抽泣声。