Placement of 了: difference between 吃飯了 and 吃了飯
From my answer to this question:


  Example cases with 了
  
  
  了: aspect marker, indicating completed action. 
  了: final particle, indicating change of situation.
  了: final particle, indicating sentence has ended; emphasize sentence; soften tone
  


我吃了早飯:


了 is an aspect marker indicating completed action when it is following a verb but not at the end of a phrase, therefore, 我吃了早飯 can only mean "I've eaten breakfast" (the verb 吃 is completed)


我吃早飯了 :


If 了 is treated as a aspect marker, the sentence would mean "I have eaten breakfast" (the verb phrase 吃早飯 is completed)
If 了 is treated as a final particle (indicating change of situation), the sentence would mean "I (now) eat breakfast (from previously not eating breakfast to now eating breakfast)
or  more likely, 了 indicates 1. the sentence has ended,  2. adding emphasis or soften the tone of the sentence


"我吃早飯" sounds blunt and make the sentence sound incomplete

In "我吃了早飯了", the first 了 indicates the verb 吃 is a completed action and the second 了 indicates the sentence is finished (also emphasize the sentence / soften the tone)