Learn Turkish: Herkes kazandığı oranda vergi öder. - Everyone pays tax in proportion to what they earn.

This uses the structure you are learning now: Verb + -dığı + oranda. First, we take kazan (to earn) and add the -dık participle plus the 3rd person singular possessive -ı, making kazandığı. Then we add oranda (at the rate). Literally, it means 'at the rate of their earning.' This is the standard Turkish way to express 'in proportion to [verb]'.

Why does kazandığı use the 3rd person singular possessive (-ı) when Herkes means 'everyone'?

In Turkish, words like herkes (everyone) and hiç kimse (no one) are treated as grammatically singular. Because of this, any verbal noun or verb relating to them takes a 3rd person singular suffix. That is why we use kazandığı instead of the plural kazandıkları.

Could I use ölçüde instead of oranda in this sentence?

Yes, you could say kazandığı ölçüde. Both oranda (from oran, meaning rate or ratio) and ölçüde (from ölçü, meaning measure or extent) are used interchangeably in these proportional clauses. Oranda feels slightly more mathematical or financial, which fits perfectly with paying taxes.

Why is the aorist tense used in öder instead of the present continuous ?