LaTeX works very hard to find optimal places to split lines of
text in making paragraphs. You can help it by indicating when it
should avoid a line break. Use a ~ for a space that should not be
used to break a line.
When shouldn't you break a line?
LaTeX inserts more space at the end of a sentence then between
its words. If you use an abbreviation like Dr. in the middle of a
sentence, then you need to let LaTeX know that the period is
not the end of a sentence, with a ~ (if the line should not be broken
there) or a \ followed by a space (if the line could be broken there).
If you need to force a line to break at a given point, use \\.