我吃了饭 ==> 了 is an auxiliary (助词) indicating a perfective aspect. 我吃了饭就去找你 我吃饭了 ==> 了 is a modal particle (语气词) expressing an affirmative mood (肯定语气). 我吃饭了就去找你 ==> Evidence that it does not indicate a tense. Yeah, why not *他死了了? Apparently there is an unspoken rule regarding the use of double 了s. I think I have been conditioned to avoid such constructions because 我吃过饭了 sounds natural but 我吃了饭了 is something unfamiliar to me. This is my question: If 我回了你的短信了= 我已经回了您的短信, the "affirmative" 了 seems to be optional. Then why is the second 了a must in 我回了你的短信了? I'm trying to figure out why the second 了 would make the expression more colloquial or idiomatic. Can I say "是的, 我回了你的短信"?