Line-breaks are usually calculated as necessary by MusiCAD. These line-breaks are called soft line-breaks. A variant on soft line-breaks occur whenever you limit the number of bars per line in layout.
However, you can force a line-break by inserting a hard (unconditional) line-break: using the bar line menu or <ctrl-Enter>
A third kind of possible line-break is the conditional line-break (the one marked with a question-mark) which will occur only if 'necessary'. Necessity comes up when a conditional line-break is encountered within the line break region.
Explicit line breaks are inserted by means of the bar line-menu
See also page-break