The non-past endings tell you who is performing the action: the speaker (called the first person: 'I, we'), the person spoken to (called the second person: 'you'), or a third person ('he/she/it' or 'they'). There are actually three versions of 'you' in Russian: the Plural вІ, the informal Singular тІ, and the formal Singular вІ.
Thus, the first person singular ending -ю in ї понимђ-ю matches the Nominative subject ї in the meaning I. The second person singular ending -шь in тІ понимђешь matches the Nominative subject тІ in the meaning you (Singular informal). And so on, for a full set of six endings. (These are treated in detail elsewhere.)