Revelation 18:11; The Merchants Shall Weep and Mourn Her

Revelation 18:11 (KJV)  And the merchants of the earth shall weep and mourn over her; for no man buyeth their merchandise any more: 

The description becomes more graphic at this point as the verb tenses changes to the present – weep and mourn or grieve. At v15, however, it reverts to the future tense to describe the same thing (Alford).

Revelation 18:15 (KJV)  The merchants of these things, which were made rich by her, shall stand afar off for the fear of her torment, weeping and wailing, 

An analogous shift in tenses occurs in the description of the two witnesses in Revelation 11:9-10.

Revelation 11:9-10 (KJV)  And they of the people and kindreds and tongues and nations shall see their dead bodies three days and an half, and shall not suffer their dead bodies to be put in graves. And they that dwell upon the earth shall rejoice over them, and make merry, and shall send gifts one to another; because these two prophets tormented them that dwelt on the earth. 

The two verbs occur together at Revelation 18:15, 19 also (cf. Luke 6:25; James 4:9). It is a picture of overwhelming sadness.

Revelation 18:19 (KJV)  And they cast dust on their heads, and cried, weeping and wailing, saying, Alas, alas, that great city, wherein were made rich all that had ships in the sea by reason of her costliness! for in one hour is she made desolate. 

Luke 6:25 (KJV)  Woe unto you that are full! for ye shall hunger. Woe unto you that laugh now! for ye shall mourn and weep.

James 4:9 (KJV)  Be afflicted, and mourn, and weep: let your laughter be turned to mourning, and your joy to heaviness. 

[Robert L. Thomas, Revelation 8-22 An Exegetical Commentary, Moody Press, 1995, 330]