Question on Performance

I've never been on the forum before .   We have a very complex query we are trying to optimize.  We did not write it, but have to fix it.   A select statement can be put into the Select Clause, the From Clause, and the Where Clause.  Is there a performance advantage to one location as opposed to other locations.  Somewhere in the back of my mind, I thought if you create select statements in the From Clause it runs once (like a temp table) and then uses the info for all the records being queried.   I appreciate your expertise on this.