Here are some examples of how it's used:
* Вечная любовь (Vechnaya lyubov) - Eternal love
* Вечное лето (Vechnoe leto) - Eternal summer
* Вечный огонь (Vechny ogon) - Eternal flame
You can also use the word навсегда (navsegda) which means "for good," "forevermore," or "for all time."
For example:
* Я люблю тебя навсегда (Ya lyublyu tebya navsegda) - I love you forever.
* Прощай навсегда (Proshchay navsegda) - Goodbye forever.
Both words are common and widely used in Russian.