  The effects of a scalar field, known as the "assistant field," which
nonminimally couples to gravity, on single-field inflationary models are
studied. The analysis provides analytical expressions for inflationary
observables such as the spectral index ($n_s$), the tensor-to-scalar ratio
($r$), and the local-type nonlinearity parameter ($f_{\rm NL}^{(\rm local)}$).
The presence of the assistant field leads to a lowering of $n_s$ and $r$ in
most of the parameter space, compared to the original predictions. In some
cases, $n_s$ may increase due to the assistant field. This revives
compatibility between ruled-out single-field models and the latest observations
by Planck-BICEP/Keck. The results are demonstrated using three example models:
loop inflation, power-law inflation, and hybrid inflation.
