If you say that something will happen or continue forever, you mean that it will always happen or continue.
I think that we will live together forever.
我想我们会永远生活在一起。
If something has gone or changed forever, it has gone or changed completely and permanently.
The old social order was gone forever.
旧的社会秩序一去不复返了。
If you say that something takes forever or lasts forever, you are emphasizing that it takes or lasts a very long time, or that it seems to.
The drive seemed to take forever.
好像永远也开不到头似的。