What are the essential conditions for a valid Nikah (Marriage contract)?

Marriage (Nikah) is a great blessing of Allah that protects one from sin. For a Nikah to be valid, two essential conditions must be met: 1. Ijab and Qabul (Proposal and Acceptance): One party must propose and the other must accept in the same sitting. 2. Witnesses: The proposal and acceptance must take place in the presence of at least two sane, adult Muslim males, or one male and two female witnesses. The witnesses must hear the words clearly. If a Nikah is performed without witnesses, it is invalid. The words used for the contract must be in the past tense (e.g., 'I have given my daughter in marriage' and 'I have accepted') or one in the imperative and one in the past. It is also a condition that the parties being married are clearly identified by name and parentage to avoid any ambiguity.