Chinese doesn't have as many past tense grammar structure as English does. And usually you will see/hear "了” when are talking in past tense. Therefore those sentences translated in Chinese are: 这个是个很好的提议啊/ at least you want to say that it was a good idea but it isn't anymore, then the translation would be: 这个本来是一个很好的提议... 我在这里了 (this sentences refers that the speaker still is there), if you want to say that the speaker is not there anymore then you have to words such as: 刚刚，刚才... But you have to omit the word "了". 这部电影很棒/这部电影很好看！we do not use this phrase in past at least you want to say that this movie is not great anymore. 他们以前是我的同事 Hope my explanation was helpful for you!