Know we (egnōkamen). Perfect active indicative, “we have come to know and still know.” See 1Jo 2:3 for “hereby” (en toutōi).
Love (tēn agapēn). “The thing called love” (D. Smith).
He for us (ekeinos huper hēmōn). Ekeinos as in 1Jo 2:6; 1Jo 3:3, 1Jo 3:5, huper here alone in this Epistle, though common in John’s Gospel (Joh 10:11, Joh 10:15; Joh 11:50, etc.) and in 3Jo 1:7.
Laid down his life (tēn psuchēn autou ethēken). First aorist active indicative of tithēmi, the very idiom used by Jesus of himself in Joh 10:11, Joh 10:17.
We ought (hēmeis opheilomen). Emphatic hēmeis again. For opheilō see 1Jo 2:6. Of course our laying down our lives for the brethren has no atoning value in our cases as in that of Christ, but is a supreme proof of one’s love (Joh 13:37.; Joh 15:13), as often happens.