Dear Anuj Gupta,
For information on the FAST theory basis, see the forum topic found here, especially my post from December 15, 2010: Coupled blade modes in FAST. To answer your direct questions:
*FAST is based on combined modal and multi-body dynamics formulation with nonlinear beam elements.
*FAST’s aerodynamics module (AeroDyn) is based on a blade-element/momentum (BEM) or generalized dynamic wake (BEM) wake model, together with a Beddoes-Leishman dynamic stall model.
*The direct wind load on the tower is not currently included, but it is something we are working to add.
Best regards,