That which each party owned prior to commencing the relationship usually belongs to that person at the end of the relationship. It is known as “excluded property”.
Only the increase in value that occurred during the relationship is “family property” and up for division. The valuation date of property is at the date of trial, not the separation date, barring any unusual facts.

Decreases in value during a relationship are not shared by the parties. The party who suffered the decrease usually gets stuck with it.
Generally, that which is put into the property by a spouse, or a parent of a spouse is returned to that person prior to division, but proof is required and has to be believed.
Assets can include such things as pension credits acquired during the relationship only (i.e., CPP) and is divided by the pension at the source at the time the pension kicks in and starts to pay out.

Debts can also be divided by the parties. The amounts have to be proven and believed. They must have been incurred for a family purpose.

Negligent dissipation of assets can affect the division of assets too (i.e., one party gambles the assets away).