Please explain how to differentiate between the cause of torsional strain & vanderwaal strain.
Steric strain is the strain resulting from non bonded atoms being forced too close together (more specifically, to within the regions of one another's Van der Waal radii, hence the alternative name of this sort of strain; "Van der Waals strain").
Tosional strain is the resistance to bond twisting.
This refers to the tendency for some molecules to stay in the anti-periplanar conformation, as opposed to having relatively free rotation around the single bond. This is generally due to bulky groups vicinal to one another, or can be to cancel any dipoles that may exist in the syn- or gauche- conformations.
For simplicity's sake,
torsional strain is defined as as the strain experienced by the bonds when conformations are not staggered. So, at any angle other than 60, 120, or 180, there is torsional strain. Additionally, torsional strain can only ever exist in atoms separated by only three bonds.
Therefore, the hydrogens in ethane only ever experiences torsional strain, and it is zero (technically minimized) when it's staggered.
Steric strain exists only in molecules who have four or more bonds, since steric strain is defined as the repulsion felt between atoms at four or more bonds separated from each other forced closer than their van der Waals radius would typically allow.
In the butane below, there is torsional strain between the two central carbons and hydrogens from each central methylene group. However, atoms separated by four or more bonds experience steric strain. Steric hindrance will never be zero, but it can be minimized when the atoms are separated by as much space as possible.