How do you create a sentential form in a given grammar?
If you know how to decide whether a word belongs to the language generated by a grammar, then you can extend this to sentential forms, in the following way. For each nonterminal $A$, add a new terminal $\mathbf{A}$. Add rules $A \to \mathbf{A}$. Given a sentential form, replace each nonterminal $A$ with the terminal $\mathbf{A}$. Now ask whether the resulting word belongs to the language generated by the augmented grammar.
As an example, your grammar turns to
\begin{align}
S &amp;\to aBS \mid abT \mid a \mid \mathbf{S} \\
T &amp;\to d \mid dT \mid \mathbf{T} \\
B &amp;\to da \mid \epsilon \mid S \mid \mathbf{B}
\end{align}
The original grammar generates $aBaabda$ iff the augmented grammar generates $a\mathbf{B}aabda$.