I read Acts 4:20 the other day, and wondered about the best way to translate the verse in to English.
Grammatically, the sentence goes like this: after the initial conjunction (γαρ) there is a negated 1pl verb which happens to be δυναμαι. As is often the case, this verb requires a complementary infinitive to make sense of the verb (e.g. “we are not able…to do something). In this case, the infinitive is also negated (with μη since it is non-indicative). Inbetween the verb and its complement is an accusative relative clause acting as the object of the infinitive (“what things we saw and heard”).
Here’s how a number of English translations tackled the construction (in no particular order):
ESV: for we cannot but speak of what we have seen and heard
NIV: For we cannot help speaking about what we have seen and heard
NET: for it is impossible for us not to speak about what we have seen and heard
NLT: We cannot stop telling about everything we have seen and heard
NRSV: for we cannot keep from speaking about what we have seen and heard
HSCB: for we are unable to stop speaking about what we have seen and heard
KJV: For we cannot but speak the things which we have seen and heard
NASB: for we cannot stop speaking what we have seen and heard