Adam Smith's four canons (1776) still open every course: equity (ability to pay), certainty (not arbitrary), convenience (easy to pay), economy (cheap to collect). Modern additions: efficiency (minimal distortion), flexibility (adaptable to cycles), simplicity (comprehensible), and productivity (adequate yield).
Burden: the direct burden is the money paid; the excess burden (deadweight loss) is the value lost by changed behaviour — the trade that did not happen because of the tax. Incidence asks who finally bears it: the statutory payer and the economic bearer may differ (the seller remits VAT; the consumer bears it).