The intensive so meaning “very or extremely” (Everything's so expensive these days) occurs chiefly in informal speech. In writing and formal speech, intensive so is most often followed by a completing that clause: Everything is so expensive that some families must struggle just to survive. The conjunction so (often followed by that) introduces clauses both of purpose (We ordered our tickets early so that we could get good seats) and of result (The river had frozen during the night so people walked across it all the next day). In formal speech and writing, so that is somewhat more common than so in clauses of purpose. Otherwise, either so or so that is standard. Like and, but1, and or, so can occur as a transitional word at the beginning of a sentence: So all our hard work finally brought results. See also as1, and, but1.