  The hadron loop effect is conjectured to be important in understanding
discrepancies between the observed states in experiments and the theoretical
expectations of non-relativistic potential model. We present that, in an easily
operable procedure, the hadron loop effect could shift the poles downwards to
reduce the differences and provide better descriptions of both the masses and
the total widths, at least, of the radial quantum number $n=1$ charmed and
charmed-strange states. The $1^1P_1-1^3P_1$ mixing phenomena could be naturally
explained due to their couplings with common channels. The newly observed $D$
states are also addressed, but there are still some problems remaining unclear.
