A.T. Robertson Commentary James 1:19

"Ye know [this], my beloved brethren. But let every man be swift to hear, slow to speak, slow to wrath:" — James 1:19 (ASV)

Ye know this (ιστε). Or "know this." Probably the perfect active indicative (literary form as in Eph 5:5; Hebrews 12:17, unless both are imperative, while in James 4:4 we have οιδατε, the usual vernacular Koine perfect indicative). The imperative uses only ιστε and only the context can decide which it is. Εστο (let be) is imperative.

Swift to hear (ταχυς εις το ακουσα). For this use of εις το with the infinitive after an adjective see 1 Thessalonians 4:9. For εις το after adjectives see Ro 16:19. The picture points to listening to the word of truth (verse 18) and is aimed against violent and disputatious speech . The Greek moralists often urge a quick and attentive ear.

Slow to speak (βραδυς εις το λαλησα). Same construction and same ingressive aorist active infinitive, slow to begin speaking, not slow while speaking.

Slow to anger (βραδυς εις οργην). He drops the infinitive here, but he probably means that slowness to speak up when angry will tend to curb the anger.