Yesterday I searched for an article I read 2 or 3 days ago (this one: https://news.ycombinator.com/item?id=30007856). I was 70% sure that the words "home" and "network" appeared in the post, so I searched in "Stories" in "Past Week" for these keywords and didn't find it.
I had to switch to "All" and noticed that the search engine didn't take the word "networking" into account which appeared in the title. I would argue that stemming and searching for root words is a basic feature of a modern search engine, if I type "site:news.ycombinator.com home network" into Google, I find the article without any problems.
So to sum up: What is the intention behind the way the search engine works? If I would type in "home network" in a full-text search engine, even that would give me the correct article but for some reason HN's search engine is not capable of that.