Under the Hindu Marriage Act, what constitutes a legitimate marriage?

The prerequisites specified in Section 5 of the Hindu Marriage Act (HMA) must be met in order for the marriage to be valid. The most crucial requirement is that both parties be Hindus as defined by Section 2 of the HMA.
 
Requirements:
A marriage between any two Hindus can be solemnized if the following conditions are met:

1. At the time of the marriage, neither party has a live spouse;
2. at the moment of marriage, neither party:
   
   1. is unable to give legitimate consent due to mental illness;
   2. has been suffering from a mental disease of such a nature or extent that he or she is unsuitable for marriage and childbearing, although being capable of giving legal consent; or
   3. has had several episodes of lunacy.
3. the bridegroom has attained the age of 21 [twenty-one years] and the bride has reached the age of 18 [eighteen years] at the time of the marriage;
4. the parties are not in a prohibitory relationship unless each of them has a custom or usage that allows them to marry;
5. The parties are not sapindas of each other until each party's custom or use approves of a marriage between them.

Explanation of the Circumstances:
The concept of monogamy is enshrined in the first criterion.
The second requirement is one of the other parties to the marriage's mental competence, and it forbids an idiot and lunatic from marrying.
To enter into a marriage, the bride and groom must be at least 18 years old, according to the third condition. This condition requires the bride and groom's consent to the marriage because the law considers them adult at a certain age.
Individuals in banned degrees are prohibited from marrying unless it is a usual practise or is established in a community's usage. The laws in each of their jurisdictions allow for a marriage between them.
The fifth criterion is similar to the fourth, except that it prohibits two sapindas from marrying each other.

What are the HMA's remedies for annulment/dissolution of marriage?

Marriages that are in contravention of Sections 5 (i) (iv), and (v) of the Hindu Marriage Act have been declared null and void from the beginning.
 
Voidable marriages are those that violate the grounds listed in Section 12 of the HMA, and such marriages may be annulled by the Court if the Section 12 requirements are met.
 
Judicial Separation: Either party to a marriage may file a petition for judicial separation under Section 10 on any of the grounds listed in subsection I of Section 13, or, in the case of a wife, on any of the grounds listed in subsection (ii) thereof, as grounds for divorce.
 
Divorce: Any marriage solemnized under the HMA may be dissolved by a decision of divorce based on a petition filed by either the husband or the wife under Section 13 of the HMA. This is a contested divorce under Section 13 of the Family Code.
 
Section 13A of the HMA provides an alternative type of redress in divorce proceedings. When a petition for dissolution of marriage is filed, the court may instead issue a judicial separation judgment. The clauses (ii), (vi), and (vii) of Section 13 would be an exception to the relief.
 

Section 13B: Both parties to a marriage may file a petition for dissolution of marriage by a decree of divorce in the District Court, and the court may grant the decree of divorce if the criteria stated in section 13B are met.

References:

• https://www.thelawcodes.com/valid-marriage-under-hindu-law/
• https://www.thelawcodes.com/lawyers-chandigarh/
• https://www.thelawcodes.com/lawyers-in-gurgaon/
• https://www.thelawcodes.com/divorce-lawyers-in-chandigarh/
• https://www.thelawcodes.com/divorce-lawyers-in-gurgaon/
• https://www.thelawcodes.com/civil-lawyers-in-chandigarh/
• https://www.thelawcodes.com/civil-lawyers-in-gurgaon/
• https://www.thelawcodes.com/lawyers-in-delhi/