A.T. Robertson Commentary Acts 20:31

"Wherefore watch ye, remembering that by the space of three years I ceased not to admonish every one night and day with tears." — Acts 20:31 (ASV)

Wherefore watch ye (διο γρηγορειτε). Paul has concluded his defence of himself and his warning. Now he exhorts on the basis of it (διο) because of which thing. The very command of Jesus concerning the perils before his return as in Mr 13:35 (γρηγορειτε), the very form (late present imperative from the second perfect εγρηγορα of εγειρω, to arouse). Stay awake.

I ceased not to admonish (ουκ επαυσαμην νουθετων). Participle describes Paul, I did not cease admonishing, night and day (νυκτα κα ημεραν, accusative of extent of time, for three years τριετιαν, accusative of extent of time also). Νουθετων is from νουθετεω, to put sense into one. So Paul kept it up with tears (verse 19) if so be he could save the Ephesians from the impending perils. Forewarned is to be forearmed. Paul did his duty by them.