In a main idea, often appears at the beginning of a paragraph or a text, it may not necessarily be there, because the main idea could be a mixture of fragments of ideas in a text distributed. Typically, the overall content of the text revolves around this idea. Also, can be found at or within text, and in some cases the main idea is not written, but implied. is all the termination.
secondary idea: The secondary idea which serves as a complement to the main idea, it gives more details. The descriptions are usually secondary ideas.
Something very important in differentiating secondary main ideas is that the former is meaningless by itself, but the other does not. If you remove a whole paragraph and just stay with the main idea, the resulting sentence would be meaningless. However, secondary ideas is not so, since they are subordinate to the principal.