One way to decide whether to use like or as is to apply grammar rules, as explained in Part 1.
Another strategy is to think about what you mean. To include what you’re comparing in the group to which you’re comparing it, use such as: Dementias such as Lewy body disease and Alzheimer’s cause progressive mental decline. In this case, both Lewy body disease and Alzheimer’s are part of the same category: dementias.
If you want to exclude the topic of your comparison from the category to which you’re comparing it, use like. For example, Symptoms of Lewy body dementia are much like those of Alzheimer’s. Using like distinguishes between these two forms of dementia.
If you’re a visual learner, you might find Edwin Markham’s description of circles in “Outwitted” helpful:
- He drew a circle that shut me out–
- Heretic, a rebel, a thing to flout.
- But Love and I had the wit to win:
- We drew a circle that took him in!
Like draws a circle that shuts out. In the example above, Alzheimer’s and other dementias would be inside the circle; Lewy body disease would be shut out.
Such as creates a circle that takes in everything in the comparison, including Alzheimer’s, other dementias, and Lewy body disease.
What should you remember? Like excludes; such as includes.
Geoff Pope summarizes experts’ opinions on like and such as in this Grammar Girl post: http://grammar.quickanddirtytips.com/like-versus-such-as.aspx
Writer’s Block distinguishes between like and such as based on meaning: http://www.writersblock.ca/tips/monthtip/tipjun98.htm
Learn more about Edwin Markham at Modern American Poetry: http://www.english.illinois.edu/maps/poets/m_r/markham/markham.htm