Article before the word useful
It isn't a period, here is a useful article: http://www.drspock.com/article/0,1510,6004,00.html
Hope the following article on jewelry trends will be useful for you.
A seminal article, or "seminal work", is one that has within it the ideas that spawn a great deal of useful research or creative production in a given field.
it could be useful by helping others with their explorations
Here is an useful article on foods that are good for manage glucose levels: http://www.livestrong.com/article/311255-good-foods-for-glucose-management/
No! It's an article. Will, have, and can are some examples of helping verbs.
They allowed for consistent readable typefaces, could make multiple copies of the same article
Visit sites dedicated to Marathon running. It would also be very useful to talk with someone who previously competed in a marathon to get useful advice.
On the website Hugh Graham Creative there are lots of useful information about design concepts for free. The article is also enriched by an informative chart and useful examples.
Use the Rydberg formula. A useful article about this is on Wikipedia. It is called "Hydrogen spectral series".
Almost any article or accessory can be made from wool -- useful by humans from infant to senior, even some useful by animals. Examples include clothing, bedding, shoes, bags and more.