Thank you, I got it.

Constant and eternal are pretty close synonyms. Eternal means forever- from the beginning til the end of time. Constant is something that keeps on happening or being but might end at any time. A constant companion- you might not be companions in the future. An eternal companion- someone you will always be with. Has been a constant companion. Vs. will be an eternal companion. The first example looks back at what has happened, where as the second looks to what will happen in the future. Another way to deal with this is just to think about the two words in terms of collocation (words that usually go together. ) Constant pain. Constant companion. Constant nagging. Constant annoyance. Constant noise. Eternal happiness. Eternal sorrow. Eternal damnation (!) eternal feeling.