73 In the tetrahedron $A-B C D$, edge $A B=a, C D=b, A B$ and $C D$ form an angle of $\theta, A B$ and $C D$ are at a distance of $d$. Then the volume of the tetrahedron $A-B C D$ is
$73 \frac{1}{6} a b d \sin \theta$. Let $E F$ be the common perpendicular of $A B$ and $C D$, and the perpendicular from $C$ to the plane $A B F$ be $C G$, with $G$ as the foot of the perpendicular, then $C G$'s ray $F G \perp E F$, so $F G / / A B, \angle C F G=\theta, O G=C F \sin \theta$,
$$
V_{C-A B F}=\frac{1}{3} \times \frac{1}{2} \times E F \times A B \times C F \sin \theta
$$

Similarly,
$$
V_{D-A B F}=\frac{1}{3} \times \frac{1}{2} \times E F \times A B \times F D \sin \theta \text {. }
$$

Adding them up, we get
$$
V_{A B C D}=\frac{1}{6} a b d \sin \theta
$$