Like DMF and formamide, each of the two rotamers of NMF are described by two principal resonance structures:
This description highlights the partial double bond that exists between the carbonyl carbon and nitrogen, which gives rise to a high rotational barrier.
 Thus, the molecule is not able to freely rotate around its main axis and the (E)-configuration is preferred due to steric repulsion of the larger substituents.
Like DMF and formamide, each of the two rotamers of NMF are described by two principal resonance structures:
This description highlights the partial double bond that exists between the carbonyl carbon and nitrogen, which gives rise to a high rotational barrier.  Thus, the molecule is not able to freely rotate around its main axis and the (E)-configuration is preferred due to steric repulsion of the larger substituents.
N-Methylformamide