  Light bridges, the bright structures that divide the umbra of sunspots and
pores into smaller pieces, are known to produce wide variety of activity events
in solar active regions (ARs). It is also known that the light bridges appear
in the assembling process of nascent sunspots. The ultimate goal of this series
of papers is to reveal the nature of light bridges in developing ARs and the
occurrence of activity events associated with the light bridge structures from
both observational and numerical approaches. In this first paper, exploiting
the observational data obtained by Hinode, IRIS, and Solar Dynamics Observatory
(SDO), we investigate the detailed structure of the light bridge in NOAA AR
11974 and its dynamic activity phenomena. As a result, we find that the light
bridge has a weak, horizontal magnetic field, which is transported from the
interior by large-scale convective upflow and is surrounded by strong, vertical
fields of adjacent pores. In the chromosphere above the bridge, a transient
brightening occurs repeatedly and intermittently, followed by a recurrent dark
surge ejection into higher altitudes. Our analysis indicates that the
brightening is the plasma heating due to magnetic reconnection at lower
altitudes, while the dark surge is the cool, dense plasma ejected from the
reconnection region. From the observational results, we conclude that the
dynamic activity observed in a light bridge structure such as chromospheric
brightenings and dark surge ejections are driven by magnetoconvective evolution
within the light bridge and its interaction with surrounding magnetic fields.
