Tax is a general word, as is levy, though levy is not used so much. Duty is more like a tariff on trade. An impost is a tariff used in retaliation for tariffs of other countries. In English, the only real difference in these words is the imagination of governments to extract money from their citizens: duty - 关税 tax - 税 impost - 以牙还牙的关税 levy - 征税