In this paper, we consider the time-frequency localization of the generator of a principal shift-invariant space on the real line which has additional shift-invariance. We prove that if a principal shift-invariant space on the real line is translation-invariant then any of its orthonormal (or Riesz) generators is non-integrable. However, for any $n\ge2$, there exist principal shift-invariant spaces on the real line that are also $\nZ$-invariant with an integrable orthonormal (or a Riesz) generator $φ$, but $φ$ satisfies $\int_{\mathbb R} |φ(x)|^2 |x|^{1+ε} dx=\infty$ for any $ε>0$ and its Fourier transform $\hatφ$ cannot decay as fast as $ (1+|ξ|)^{-r}$ for any $r>1/2$. Examples are constructed to demonstrate that the above decay properties for the orthormal generator in the time domain and in the frequency domain are optimal.