[Grammar] around which

[kadioguy]

In the book Six-Way Paragraphs, it says:

[FONT=Tahoma, Calibri, Verdana, Geneva, sans-serif]Finding Meaning. Actually, there is nothing especially important in recognizing or inferring a topic sentence for its own sake. The important thing is that the reader use the topic sentence as a quick means of establishing a focal point around which to cluster the meanings of the subsequent words and sentences that he or she reads. Here's the double-edged sword again: just as writers use topic sentences to provide focus and structure for presenting their meaning, so the perceptive reader can use the topic sentence for focus and structure to gain meaning.

[/FONT]Up to this point, the reader, having looked secretly over the writer's shoulder, should have learned two exceedingly valuable secrets: first, expect only one main idea in each paragraph; and secondly, use the topic sentence to discover the topic of each paragraph.
--------------------
1. What does 'around which' mean?
2. Why not '...provide focus and structures'?

 

[GoesStation]

1. The author thinks the topic sentence becomes a central hub, or focal point. Subsequent sentences cluster around it; it is the focal point around which​ they cluster.
2. If you unpack the phrase you get writers use topic sentences to provide focus and to provide structure. Does that help?

 

[kadioguy]

2. If you unpack the phrase you get writers use topic sentences to provide focus and to provide structure. Does that help?

I thought the word focus is in the plural, and this is why I thought the word structure should also be structures. After looking up the dictionary, now I find focus is in the singular.