Questions & Answers about Лікар пояснив дозу й побічний ефект.
Лікар is in the nominative singular because it is the subject of the sentence: the person who performed the action.
- лікар = doctor
- In this sentence: The doctor explained...
A useful detail: лікар is a masculine noun, even though it ends in a consonant and not in -р specifically. Many masculine Ukrainian nouns end in a consonant.
Пояснив is the past tense, masculine singular form of the verb пояснити (to explain).
It agrees with the subject:
- лікар = masculine singular
- so the verb is пояснив
Compare:
- лікар пояснив = the doctor explained
- лікарка пояснила = the female doctor explained
- лікарі пояснили = the doctors explained