Consent makes the marriage. Every ground of nullity is, in the end, a way in which that consent may be absent, defective, or directed at something other than marriage.
What follows is illustrative only. It does not exhaust the grounds provided by the Code, and it is not a catalogue into which a case is to be fitted: these are the heads that arise most often. Which ground to plead, and whether more than one, is a judgement made on the facts — and it is the choice that bears most on the outcome.
Incapacity to consent (can. 1095)
This is the most frequent and the most complex ground. The canon distinguishes three cases: lack of sufficient use of reason; grave lack of discretion of judgement concerning the essential matrimonial rights and duties to be mutually given and accepted; and inability to assume the essential obligations of marriage for causes of a psychic nature.
The latter two do not require a diagnosed psychiatric illness. What must be proved is a condition existing at the time of the wedding such as to prevent a mature judgement about the bond, or the assumption of the obligations that flow from it. Most causes are decided on this ground, and the court-appointed expert report is decisive.
Simulation (can. 1101)
Simulation arises where one or both parties, by a positive act of the will, exclude marriage itself or one of its essential elements or properties: indissolubility, fidelity, ordination to the good of the spouses and to procreation, sacramental dignity. A general intention or an opinion is not enough: an actual, contrary and demonstrable act of the will is required.
Deceit (can. 1098)
A marriage is invalid if it is contracted as a result of deceit, perpetrated in order to obtain consent, concerning some quality of the other party which of its nature can gravely disturb the partnership of conjugal life.
Error (can. 1097)
The canon draws a distinction that is easily missed. Error about the identity of the person — § 1 — invalidates the marriage outright. Error about a quality of the person — § 2 — does not, even where it gave rise to the contract, unless that quality was directly and principally intended. It is not enough that the quality determined the choice: the will must have been directed to it before the person. Hence a head that looks broad is narrow in practice, and distinct from deceit, which operates where the quality was concealed or misrepresented.
Distinct again is determining error concerning unity, indissolubility or the sacramental dignity of marriage, which can. 1099 treats as relevant where it determined the will.
Force and fear, condition
Relevant here is marriage contracted through force, or through grave fear inflicted from outside such that the party is compelled to choose marriage to escape it; and marriage contracted subject to a condition.
Impediments and defect of form
Alongside defects of consent there operate the diriment impediments and the defect of the prescribed canonical form. These arise less often in practice but are more straightforward to establish.