A.T. Robertson Commentary James 4:15

"For that ye ought to say, If the Lord will, we shall both live, and do this or that." — James 4:15 (ASV)

For that ye ought to say (αντ του λεγειν υμας). "Instead of the saying as to you" (genitive of the articular infinitive with the preposition αντ and the accusative of general reference with λεγειν), "instead of your saying."

If the Lord will (εαν ο κυριος θελη). Condition of the third class with εαν and the present active subjunctive (or first aorist active θελεση in some MSS). The proper attitude of mind (Acts 18:21; 1 Corinthians 4:19; 1 Corinthians 16:7; Romans 1:19; Php 2:19,24; Hebrews 6:3), not to be uttered always in words like a charm. This Hellenistic formula was common among the ancient heathen, as today among modern Arabs like the Latin deo volente.

This or that (τουτο η εκεινο). Applicable to every act.