Quasi-bound states in the continuum (q-BICs) provide an effective approach for achieving strongly confined resonances in metasurfaces. However, the simultaneous realization of high-Q, polarization-multiplexed, and tunable resonances at a single frequency using a single resonant meta-atom remains challenging. Here, we numerically design and investigate a tunable all-dielectric terahertz (THz) metasurface consisting of a single asymmetric germanium (Ge) resonator by employing COMSOL Multiphysics. By introducing only one asymmetry parameter, the symmetry-protected BIC is transformed into a radiative q-BIC under two orthogonal linear polarizations. Through the coupling between the q-BIC and the intrinsic resonant mode, distinct interference responses are generated. Specifically, at the same frequency of 1.227 THz, a Fano resonance (Q = 606) is obtained under TE polarization, whereas an electromagnetically induced transparency (EIT)-like resonance (Q = 1105) appears under TM polarization. Owing to the tunable property of Ge, the two resonances can be actively modulated, achieving modulation depths of 96.7% and 99.1%, respectively. Furthermore, under TM polarization, the group delay can be continuously tuned from 0 to 259 ps by controlling the Ge conductivity. The proposed metasurface provides a compact platform for polarization-multiplexed and dynamically tunable THz photonic devices.