This forum is too limited to tell you all about the elements you are missing. I will give you three keywords so hopefully you can teach yourself with them.
1. Vowel Harmony in Korean, and how it affects verb conjugations and adjective declensions.
2. Vowel Contraction, such as how 'ㅣ' and 'ㅏ' become 'ㅑ', 'ㅣ' and 'ㅓ' become 'ㅕ', especially in verb conjugations.
3. Korean suffixes. I guess this one can be explained here though...
I am just taking the answer from TalkToMeInKorean website. They are saying the same as Mjkang already explained. The TTMIK teachers said that 아서/어서/여서 cannot be used with imperative sentences or with "Let's".
You can check there lesson 14 level 5. It's a welldone site. I recommend it.