There are two different counting systems in Welsh. One of these is the
more recent decimal system. In this system, un is
combined with deg ("ten") and a second number to make up the "teen"
numbers. A number like 17 (un deg saith) is literally
"one ten (and) seven".
In English, you can use one as a pronoun to
refer impersonally to yourself or to people in general: One may talk
about oneself. In Welsh, however, un is not used in the same way; you
wouldn't usually say Mae un yn siarad amdano'i hun. In more old-fashioned
Welsh, you might find people using dyn to perform this
function: Mae dyn yn mwynhau dysgu -
A man enjoys learning or
One enjoys learning. A more inclusive
choice is to use the indefinite pronoun rhywun, which is
equivalent to English "someone": Mae rhywun yn adnabod ei
ffrindiau - One knows one's
friends (literally Someone knows
his/her friends.).