Progressive tax program
If something is mandatory then use private setters and constructor arguments, and validate in the constructor (TaxLevel) [1];
Use collection initializer notation [2];
DeterminateTaxLevels should be private and called from the constructor (I would actually make it a simple attribution in the constructor);
DeterminateTaxLevels should have a better name like LoadTaxLevels, because it does not &quot;determine&quot; anything;
DeterminateTaxLevels has no reason to return bool - why does it?;
Salary is misspelt;
Either call the salary &quot;bruto&quot; or &quot;in Nis&quot; - you should be consistent, and I advise &quot;bruto&quot; because it means more than some random TLA like NIS (whatever it is);
As @Gene S said, try to follow coding standards: use brackets in ifs, use line-breaks after ifs, use camelCase on parameter names and not PascalCase, etc...


footnotes
[1]:
private class TaxLevel
{
    /// &lt;summary&gt;
    /// The money count that the tax applies to.
    /// &lt;/summary&gt;
    public decimal Money { get; private set; }
    /// &lt;summary&gt;
    /// The tax level where 1.0 is 100% tax.
    /// &lt;/summary&gt;
    public decimal TaxDeduction { get; private set; }
    
    public TaxLevel(decimal money, decimal taxDeduction) {
        if (money &lt; 0 ) {
            throw new ArgumentOutOfRangeException(&quot;Money must be grater than 0&quot;);
        }
        if (taxDeduction &lt; 0 || taxDeduction &gt; 1.0m) {
            throw new ArgumentOutOfRangeException(&quot;TaxDeduction must be between 0.0 and 1.0&quot;);
        }
        Money = money;
        TaxDeduction = taxDeduction;
    }
}

[2]:
_TaxLevels = new List&lt;TaxLevel&gt;() {
    new TaxLevel {Money = 4770m, TaxDeduction =  0.10m },
    new TaxLevel { Money = 3700, TaxDeduction = 0.14m },
    // ...
    new TaxLevel { Money = decimal.MaxValue, TaxDeduction = 0.42m },
};