Wolfram Research, Inc.
A.2.10 The Extent of Input Expressions
Mathematica will treat all input that you give on a single line as being part of the same expression.
Mathematica allows a single expression to continue for several lines. In general, it treats the input that you give on successive lines as belonging to the same expression whenever no complete expression would be formed without doing this.
Thus, for example, if one line ends with =, then Mathematica will assume that the expression must continue on the next line. It will do the same if for example parentheses or other matchfix operators remain open at the end of the line.
If at the end of a particular line the input you have given so far corresponds to a complete expression, then Mathematica will normally begin immediately to process that expression.
You can however explicitly tell Mathematica that a particular expression is incomplete by putting a \ or a (\[Continuation]) at the end of the line. Mathematica will then include the next line in the same expression, discarding any spaces or tabs that occur at the beginning of that line.
If you are using StandardForm input in a notebook front end, then Mathematica will also not treat an expression on a particular line as being complete if the line that follows it could not be complete without being combined with its predecessor. Thus, for example, if a line begins with an infix operator such as or /, then Mathematica will combine this line with the previous one to try to obtain a complete expression. If a line begins with , , , or another operator that can be used both in infix or prefix form, then Mathematica will still combine the line with the previous one, but will issue a warning to say what it is doing.