The Greek phrase that is translated as "for ever" or "forever" is εἰς τὸν αἰῶνα: (eis ton aiona) meaning "until the era." Jesus uses this phrase eleven times. It is somewhat consistently translated as "for ever," but it doesn't mean that.
"Era" is from aiôn, which means "lifetime," "life," "a space of time," "an age," an epoch," and "the present world." This phrase does not mean "forever" or anything like it. It specifically means in a limited span of time. Jesus uses it eleven times in this form. The word translated as "for" (eis) can mean "for" but "for a purpose" not "for" a span of time. When used with a time, it means "until," "as far as," or "up to." Its most common use is "into" a place.
The Greek word, , aiôn, was a measurement of time like we talk about "a generation," that is, the space between one generation and the next, which is around thirty years. The word aion can refer to longer period of time, but it is not the word for "eternity" or anything like it. In modern Greek, this word has become the basis for the Greek words for "eternity," aioniotes (αιωνιότης) and aionioteta (αιωνιότητα) but not in ancient Greek. These concepts were taken from the Bible, making their way into the modern Greek language.
The ancient Greeks used the words "for all ages," using the plural of aion, to come closer to describing"eternity," or more simply "for ages." Another word, athanatos, (ἀθάνατος) literally meaning "not dying" is much more commonly used to express this idea as "undying" and "immortal." This word was very common in ancient Greek, used to describe the perpetual existence of the Greek gods. However, we must remember that even the Greek gods were not seen as lasting for eternity. They could be destroyed even though they did not die. The gods themselves had destroyed their own parents, the Titans.
The ancient people in general did not have the same idea of "eternity" that we seem to have now. They saw the world as constantly changing. Nothing was unchanging, not even the gods, who could be destroyed and changed. From my study of ancient Chinese, I never ran into the concept of "eternity." Indeed, much ancient Chinese philosophy holds that nothing can last for all time or outside of time.
We also see this inability to express the idea of "eternal" among ancient Judeans. The word "eternity" only appears once in the KJV translation of the Bible, in Isaiah 57:15. The Hebrew word is 'ad, עַדI which means "perpetually," and "into the future" is translated as "eternity." However, more generally, it has the sense of "ongoing," which is probably a pretty good way to describe the Greek aionios. In the Greek Septuagint, this was rendered as "an age of holy awe," which is kind of a cool way to describe a long time. Despite the focus on "eternity" in more modern Christianity, this is as close as the KJV translators could honestly come to finding the word in the Hebrew books.