The English phrase "I will greatly mutliply" (A.V.) is translated from the Hebrew phrase הַרְבָּה אַרְבֶּה (harbah arbeh). This Hebrew phrase consists of two verbs, both in binyan Hif'il, the former being an infinitive absolute, while the latter is in the imperfect tense. This is a frequent Semiticism in the Hebrew Tanakh, and it should not be translated literally into English.

Regarding this particular construction, Wilhelm Gesenius wrote,¹

(a) The infinitive absolute used before the verb to strengthen the verbal idea, i.e. to emphasize in this way either the certainty (especially in the case of threats) or the forcibleness and completeness of an occurrence. In English, such an infinitive is mostly expressed by a corresponding adverb, but sometimes merely by putting greater stress on the verb; e.g. Gn 2¹⁷ מוֹת תָּמוּת‎ thou shalt surely die, cf. 18:10, 18, 22:17, 28:22, 1 S 9⁶ (cometh surely to pass); 24:21, Am 5⁵, 7¹⁷, Hb 2³, Zc 11¹⁷; with the infinitive strengthened by אַךְ‎ Gn 44²⁸ (but 27:30 and Jacob was yet scarce gone out, &c.); Gn 4³³ הָעֵד הֵעִד בָּ֫נוּ‎ he did solemnly protest unto us; 1 S 20⁶ נִשְׁאֹל נִשְׁאַל‎ David earnestly asked leave of me; Jos 17¹³, Ju 1²⁸ וְהוֹרֵישׁ לֹא הֽוֹרִישׁוֹ‎ and did not utterly drive them out; especially typical instances are Am 9⁸ I will destroy it from off the face of the earth אֶ֫פֶס כִּי לֹא הַשְׁמֵיד אַשְׁמִיד וג׳‎ saving that I will not utterly destroy, &c.; Jer 30¹¹ and will in no wise leave thee unpunished; cf. further Gn 20¹⁸, 1 K 3²⁶, Jo 1⁷, Jb 13⁵.

Again, it is used "to emphasize...the certainty (especially in the case of threats) or the forcibleness and completeness of an occurrence."

I believe the A.V. translates appropriately as emphasizing the magnitude of multiplying, hence, "I will greatly multiply..."

The English words "I will" are properly translated from the Hebrew verb אַרְבֶּה (arbeh) which is conjugated in 1^st person ("I") and imperfect tense ("[I] will"). Think of the imperfect tense as English "future tense" in this case.

¹ Wilhelm Gesenius. [i]Gesenius' Hebrew Grammar[/i], p. 342, §113n.

The English phrase "I will greatly mutliply" (A.V.) is translated from the Hebrew phrase הַרְבָּה אַרְבֶּה (harbah arbeh). This Hebrew phrase consists of two verbs, both in binyan Hif'il, the former being an infinitive absolute, while the latter is in the imperfect tense. This is a frequent Semiticism in the Hebrew Tanakh, and it should not be translated literally into English.

Regarding this particular construction, Wilhelm Gesenius wrote,¹

(a) The infinitive absolute used before the verb to strengthen the verbal idea, i.e. to emphasize in this way either the certainty (especially in the case of threats) or the forcibleness and completeness of an occurrence. In English, such an infinitive is mostly expressed by a corresponding adverb, but sometimes merely by putting greater stress on the verb; e.g. Gn 2¹⁷ מוֹת תָּמוּת‎ thou shalt surely die, cf. 18:10, 18, 22:17, 28:22, 1 S 9⁶ (cometh surely to pass); 24:21, Am 5⁵, 7¹⁷, Hb 2³, Zc 11¹⁷; with the infinitive strengthened by אַךְ‎ Gn 44²⁸ (but 27:30 and Jacob was yet scarce gone out, &c.); Gn 4³³ הָעֵד הֵעִד בָּ֫נוּ‎ he did solemnly protest unto us; 1 S 20⁶ נִשְׁאֹל נִשְׁאַל‎ David earnestly asked leave of me; Jos 17¹³, Ju 1²⁸ וְהוֹרֵישׁ לֹא הֽוֹרִישׁוֹ‎ and did not utterly drive them out; especially typical instances are Am 9⁸ I will destroy it from off the face of the earth אֶ֫פֶס כִּי לֹא הַשְׁמֵיד אַשְׁמִיד וג׳‎ saving that I will not utterly destroy, &c.; Jer 30¹¹ and will in no wise leave thee unpunished; cf. further Gn 20¹⁸, 1 K 3²⁶, Jo 1⁷, Jb 13⁵.

Again, it is used "to emphasize...the certainty (especially in the case of threats) or the forcibleness and completeness of an occurrence."

I believe the A.V. translates appropriately as emphasizing the magnitude of multiplying, hence, "I will greatly multiply..."

The English words "I will" are properly translated from the Hebrew verb אַרְבֶּה (arbeh) which is conjugated in 1^st person ("I") and imperfect tense ("[I] will"). Think of the imperfect tense as English "future tense" in this case.

¹ Wilhelm Gesenius. Gesenius' Hebrew Grammar, p. 342, §113n.