  We find the entire form of the amplitude of two fermion strings (with
different chirality), a massless scalar field and one closed string
Ramond-Ramond (RR) in IIA superstring theory which is different from its IIB
one. We make use of a very particular gauge fixing and explore several new
couplings in IIA. All infinite $u$- channel scalar poles and $t,s$- channel
fermion poles are also constructed. We find new form of higher derivative
corrections to two fermion two scalar couplings and show that the first simple
$(s+t+u)-$ channel scalar pole for $p+2=n$ case can be obtained by having new
higher derivative corrections to SYM couplings at third order of $\alpha'$. We
find that the general structure and the coefficients of higher derivative
corrections to two fermion two scalar couplings are completely different from
the derived $\alpha'$ higher derivative corrections of type IIB.
