Learn Turkish: Vergi, kazanç ile orantılıdır. - Tax is proportional to earnings.

In Turkish, things are proportional with each other, not to each other. The adjective orantılı (proportional) almost always pairs with ile (or its attached suffix forms -la / -le). So "proportional to earnings" naturally becomes kazanç ile orantılıdır.

How does orantılıdır relate to the word oranda that we are learning in this lesson's grammar structures?

They both come from the root oran, meaning "ratio" or "rate". While this lesson focuses on using oranda to mean "in proportion to" or "at the rate of" after verbs, this sentence shows the related adjective form. Orantı means "proportion", -lı makes it "proportional", and the -dır suffix adds the factual "is" used for stating a general rule.

Could I use gelir or maaş instead of kazanç here?

Yes, depending on the exact meaning you want. Gelir means "income" and is standard in legal contexts (like gelir vergisi, income tax). Maaş specifically means a fixed "salary". Kazanç is broader, meaning "earnings", "gains", or "profits", making it a great general term for any taxable money made.

Why is there a comma after Vergi?