恐怕 kǒngpà 'I'm afraid'
As in English, this is often used to indicate 'perhaps, probably etc.'.
上课已经半个小时了，大卫还没来，他恐怕是病了。 Shángkè yǐjing bàn ge xiǎoshǐ le, Dàwèi hái méi lái, tā kǒngpà shi bìng le.
'The class started half an hour ago， David has not come yet, I'm afraid / he is probably sick.'
我最近很忙，恐怕不能来参加你的生日聚会。 Wǒ zuìjìn hěn máng, kǒngpà bù néng lái cānjiā nǐ de shēngrì jùhuì.
'I have been very busy recently; l am afraid/perhaps I won’t be able to come to your birthday party.'