永远 (yǒng yuǎn) is an adverb meaning 'forever, always' — it describes an action or state that lasts indefinitely into the future. It is more common in everyday speech than the formal, written adjective 永恒 (yǒng héng 'eternal'), which describes a quality of something. 始终 (shǐ zhōng) means 'from start to finish' and focuses on a continuous period, not an endless future.