What does "forever" really mean?
The Arabic word for "forever" that Allah Almighty used in the Noble Quran is "Abada". Abada is derived from "Abad" which means "ever". The addition of the "a" at the end of Abad makes the Arabic word be referring to or connected with something.
So Abada = forever = for-ever.
"ever" is something created by GOD. Almost everything Allah Almighty Created has a limit in time.
Allah Almighty defined "forever" for Paradise as an on going never ending thing, while the "forever" used for Hell had a time limit. Let us look at the following from the Noble Quran:
"Those who are wretched shall be in the Fire: There will be for them therein (nothing but) the heaving of sighs and sobs: They will dwell therein for all the time that the heavens and the earth endure, except as thy Lord willeth: for thy Lord is the (sure) accomplisher of what He planneth. And those who are blessed shall be in the Garden: They will dwell therein for all the time that the heavens and the earth endure, except as thy Lord willeth: a gift without break. (The Noble Quran, 11:106-108)"
Notice as brother Abdul Haleem emphasized above that the time limit that Allah Almighty had set for Paradise is endless and is a "gift without break", while the time for Hell is limited to "the time that the heavens and the earth endure, except as thy Lord willeth". It is clear that the time in Hell in the Noble Quran is limited and the time in Paradise never ending.
Notice the assurance of Allah Almighty about the time of Paradise being infinite, while He didn't make the same claim about the time of Hell. In fact, He left the time of Hell open and unclear, which clearly proves that Hell will eventually be empty according to Islam, or to say the least, it will not be infinite in time as Paradise.
Back to 'Hell will eventually be empty according to Islam' section.