3:actually 吃好了also means吃完了,but we always use 吃好了 when we are refering to finish meals instead of eating apples.
4:个means a, an or one, but we can't use 个 separately，you must put a number before个.E.G 一个，两个，三个 and you can't put two 个 together,there is no things as such.
5:说can only be used as a verb,you can choose either 话 or 说话 to use as nouns,but not 说for example
what did you say说
i heard what you said(你的话 or 你的说话)