The word(s) "warn" appears 140 time(s) in 128 verse(s) in Quran in Abdullah Yusuf Ali translation.

(31) And the Unbelievers say: "Why is not a sign sent down to him from his Lord?" But thou art truly a warner, and to every people a guide.

(32) So warn mankind of the Day when the Wrath will reach them: then will the wrong-doers say: "Our Lord! respite us (if only) for a short term: we will answer Thy call, and follow the messengers!" "What! were ye not wont to swear aforetime that ye should suffer no decline?

(33) Here is a Message for mankind: Let them take warning therefrom, and let them know that He is (no other than) One Allah: let men of understanding take heed.

(34) And say: "I am indeed he that warneth openly and without ambiguity,"-

(35) He doth send down His angels with inspiration of His Command, to such of His servants as He pleaseth, (saying): "warn (Man) that there is no god but I: so do your duty unto Me."

(36) And We gave (Clear) warning to the Children of Israel in the Book, that twice would they do mischief on the earth and be elated with mighty arrogance (and twice would they be punished)!

(37) When the first of the warnings came to pass, We sent against you Our servants given to terrible warfare: They entered the very inmost parts of your homes; and it was a warning (completely) fulfilled.

(38) If ye did well, ye did well for yourselves; if ye did evil, (ye did it) against yourselves. So when the second of the warnings came to pass, (We permitted your enemies) to disfigure your faces, and to enter your Temple as they had entered it before, and to visit with destruction all that fell into their power.

(39) Who receiveth guidance, receiveth it for his own benefit: who goeth astray doth so to his own loss: No bearer of burdens can bear the burden of another: nor would We visit with Our Wrath until We had sent an messenger (to give warning).

(40) And We refrain from sending the signs, only because the men of former generations treated them as false: We sent the she-camel to the Thamud to open their eyes, but they treated her wrongfully: We only send the Signs by way of terror (and warning from evil).