The most used word

There are two most common and very important Words -- a and the, for learners of the English language.

A means one ( not any particular one, but one in general ). For example, a classroom is normally located in a school building.

The also means one ( a definite, particular one -- you know which one, the one we've introduced earlier and perhaps been talking about several times ). For example, the classroom that my study group meets in Tuesday evenings, is on the third floor.