"Always" vs. "forever"

Solution 1:

In some contexts they mean the same, but differ syntactically...

I will always love you. I will love you forever. /not/ ?I will forever love you. 1

In other contexts there's a difference in that always usually means continuously, at all [relevant] times, whereas forever usually means for an infinite amount of time into the future. Thus...

He always catches the bus at 8am. /not/ *He forever catches the bus at 8am.

The universe may not last forever. /not/ ?The universe may not last always.

...where some people may find that last construction more acceptable than others.

Here's a useful summary of usages for always, only one or two of which overlap with forever.

_{1: The song I Will Forever Love You does exist, but it's nowhere near as well-known as I Will Always Love You, written and first recorded by Dolly Parton, massively popularised by the late Whitney Houston.}