Gap filling and multiple choice items should cause no problems. The important thing when scoring other types of item is to be clear about what each item is testing, and to award points for that only. There may be just one element, such as subject-pronoun-verb inversion, and all available points should be awarded for that; nothing should be deducted for non-grammatical errors, or for errors in elements of grammar which are not being tested by the item. For instance, a candidate should not be penalised for a missing third person -s when the item is testing relative pronouns; opend should be accepted for opened, without penalty.
If two elements are being tested in an item, then points may be assigned to each of them (for example present perfect form and since with past time reference point). Alternatively, it can be stipulated that both elements have to be correct for any points to be awarded, which makes sense in those cases where getting one element wrong means that the student does not have full control of the structure. For items such as these, to ensure scoring is valid and reliable careful preparation of the scoring key is necessary.