troff/nroff, breaking and filling

breaking

The following troff commands cause a break (end of a line):

.bp
Begin a new page
.br
Break
.ce
Center lines
.fi
Start filling
.nf
Stop filling
.sp
Vertical space
.in
Indent left margin
.ti
Temporary indent the left margin of the next line.
Use ` (backquote) in place of the leading . (dot) to suppress the break.
(blank lines)
      and leading spaces
are printed as they appear. That is, leading white space ends the previous line.
wo\p rd
causes a break at the end of word, and has the line up to, including, word spread out to fill its line.
(eof)
causes a break.

not breaking

A trailing \c in a line glues this line to the next text line; there can be control lines in between.

adjusting margins

.ad key
turns on adjustment according to key, or restores the previous adjustment if no key is given. Keys are:
l
Left-justified
r
Right-justified
c
Center
b,n
Block
The current adjustment is in the number register .j. It's not a number, I don't know what it's doing in a number register, but there you have it.

.na
does the same as .ad l, but doesn't affect the number register .j.

filling

.nf
turns it off,
.fi
 turns it on, and the number register
.u
 is 1 in fill mode, else 0.