"main condition" is correct and usable in written English.
You can use it to refer to the most important or essential element of a plan, agreement, or process. For example: "The main condition of the contract was that both parties must agree to the terms before signing."
The main condition holding them back, Desmond argues, is rent.
The main condition related to the dentine alone is Dentinogenesis imperfecta II (hereditary opalescent dentine).
The United States had made the release of the prisoners the main condition for ending the remaining sanctions.
By pledging to allow no terrorist activity from its territory, Pakistan met a main condition of India's.
Quasi-steady state of desalination process is the main condition to obtain such data.
The State Department spokesman, Richard A. Boucher, today repeated that a main condition for American military involvement was for Liberia's president, Charles G. Taylor, to quit the country.
And tautology is not just a side-effect of Whiteread's approach; it has been the main condition of her sculptures, it seems to me, from the start.
And the main condition, if we're going to be on the coexistence curve, is that the chemical potentials have to be equal.
The main condition is that the city must seek to reduce traffic through some kind of pricing mechanism, much as Mayor Michael Bloomberg envisioned in his original congestion-pricing plan.
Salim Hashmi, a spokesman for Hizbul Mujahedeen in Islamabad, said the attack was an act of frustration, caused by India's unwillingness to meet his group's main condition for peace talks: that Pakistan be included at the table.
An independent assessment of diversifying the Caribbean tax haven's revenue base is the main condition stipulated by the Foreign Office for allowing the Caymans to borrow from banks CI$50m£38m8m) immediately together with a further CI$229m loan.
