Face only vertex format is..
A valid vertex index starts from 1 and matches the corresponding vertex elements of a previously defined vertex list. Each face can contain three or more vertices.
f v1 v2 v3 ….
Face with texture and normal is..
Optionally, normal indices can be used to specify normal vectors for vertices when defining a face. To add a normal index to a vertex index when defining a face, one must put a second slash after the texture coordinate index and then put the normal index. A valid normal index starts from 1 and matches the corresponding element in the previously defined list of normals. Each face can contain three or more elements.
f v1/vt1/vn1 v2/vt2/vn2 v3/vt3/vn3 ….
And Face with normal is
As texture coordinates are optional, one can define geometry without them, but one must put two slashes after the vertex index before putting the normal index.
f v1//vn1 v2//vn2 v3//vn3 …