Learn Turkish: Çorba çok sıcak, onu tadamazsın. - The soup is very hot, you cannot taste it.

It combines the root tat (taste) with the inability suffix -a and the negative aorist suffix -maz. The -a right after the root is what adds the cannot meaning. Finally, -sın is the personal suffix for you. Without the -a, it would simply be tatmazsın (you do not taste).

I know the verb for to taste is tatmak. Why is there a 'd' in tadamazsın?

This is an example of consonant mutation. When the root tat gets a suffix starting with a vowel, like the inability suffix -a, the hard t softens into a d.

Why do we use onu here instead of just o for it?

The soup is the direct receiver of the action, making it a specific direct object. We take o (it), add the buffer letter n, and then the accusative case suffix -u to show it is the specific thing being tasted.

Could I use the present continuous tense and say tadamıyorsun instead of tadamazsın?

Yes, you could. Onu tadamıyorsun focuses on the exact present moment. However, the negative aorist tadamazsın is very common in Turkish for stating a strong conclusion about a situation: because the soup is too hot, it is impossible for you to taste it.