  Physical processes that govern the star and planet formation sequence
influence the chemical composition and evolution of protoplanetary disks. To
understand the chemical composition of protoplanets, we need to constrain the
composition and structure of the disks from whence they are formed. We aim to
determine the molecular abundance structure of the young disk around the TMC1A
protostar on au scales in order to understand its chemical structure and any
possible implications for disk formation. We present spatially resolved Atacama
Large Millimeter/submillimeter Array observations of CO, $HCO^{+}$, HCN, DCN,
and SO line emission, as well as dust continuum emission, in the vicinity of
TMC1A. Molecular column densities are estimated both under the assumption of
optically thin emission from molecules in LTE as well as through more detailed
non-LTE radiative transfer calculations. Resolved dust continuum emission from
the disk is detected between 220 and 260 GHz. Rotational transitions from
HCO$^{+}$, HCN, and SO are also detected from the inner 100 au region. From the
derived $HCO^{+}$ abundance, we estimate the ionization fraction of the disk
surface and find values that imply that the accretion process is not driven by
the magneto-rotational instability. The molecular abundances averaged over the
TMC1A disk are similar to its protostellar envelope and other, older Class II
disks. We meanwhile find a discrepancy between the young disk's molecular
abundances relative to Solar System objects. Abundance comparisons between the
disk and its surrounding envelope for several molecular species reveal that the
bulk of planet-forming material enters the disk unaltered. Differences in HCN
and $H_2 O$ molecular abundances between the disk around TMC1A, Class II disks,
and Solar System objects trace the chemical evolution during disk and planet
formation.
