What is the main idea of this paragraph from Valcanoes section of the Ready. gov website?

quite an interesting way to teach topic sentences, because the answer to this question is the best definition of a topic sentence that one can present, it is as if answering the questions taught you how to do it, this is some real teaching, it is terribly well-done.

the topic sentence in this paragraph is:

topic sentences writers stay on track and keep a paragraph from becoming an aimless collection of sentences about an idea.

which incidentally tells you what a topic sentence should be.

a phrase is a set of word that make a sense, an expression formed by two or more words.

" teacher´s dictum on the possession of human "

" made a pile practicing "

" lived to an impressive age and dies "

a clause, in a grammar way is a series of words that have a complete meaning.

" was irritated by the persecution of those who called themselves "