En el trabajo
I frequently review the etymology when learning new words, but yet to find a resource that helps me understand sentence structure. I'm currently using Rosetta Stone and the answers are obvious, but the reason is left up to me to find out.
When someone is going to work, we say "Él va a trabajar." This makes sense. He goes to work. But when they are at work we say, "Él está en el trabajo." Why do we say He is at the work? Thanks!!
4 Answers
Trabajar is the verb to work
Él va a trabajar- he is going to work. I believe it is also possible to say Él va al trabajo, as well, using the noun, but note I still use the article.
Trabajo- noun for work, job.
Él está en el trabajo. He is at work. But we could say he is on the job. He is at his job.
Why do I use no article for work, but need something for job. I can't answer that any more than why it sounds better to include it with trabajo. It's just how I hear it.
Things are just most naturally said certain ways in one language, that may be different in the other. I think the point of Rosetta is to expose you to natural phrasing without trying to have a preconception about how something is phrased in English. Often native Spanish speakers will say that doesn't sound like Spanish it sounds like a word for word translation of English. Don't try to phrase things the way we do in English, learn how it is done in Spanish.

About the matter of trabajo: To indicate that someone will go to the place of employment, we say "Va al trabajo / al despacho / al taller" because "Va a trabajar" means he is going to start the action of working, which could be anything happening anywhere. This could mean the work place, but it´s not always clear.
We almost always use "the" in English with this construction.
He is at the restaurant. He is at the park. He is at the movies. He is at the theater. He is at the mall and hundreds or thousands more.
At work, at school, at home are in fact "exceptions" in English. There are very, very few that don't take "the".
Spanish, on the contrary, follows the rule.
Both Spanish and French use the definite article much more than English does. It's just part of the difference in structure. There are tremendous differences in grammar--word position, use of articles, direct, indirect, and reflexive pronouns. Then there is Russian--a language that doesn't use articles. It is a problem.
Here's a link to a good teaching on the use of the definite article in Spanish that may help you at least a little.