so

The leanest words in English have the heaviest workloads; so—an adverb, adjective, conjunction, and pronoun—is a good example of a small but efficient dynamo.

In each of its capacities it has a different job.

As an adverb it can intensify (You look so happy!) but its typical role is to indicate a condition, degree, or extent, often one already referred to: I was disappointed and I told them so. We got there early but so did they.

When so joins clauses it usually introduces a reason or result: Shut up so I can hear. He didn’t feel well so he stayed home.

The greatest shorthand feat for so is to stand in for an entire predicate phrase in a later clause: If you have not already purchased your tickets you are advised to do so immediately.