Questions & Answers about Моя оценка по математике ниже, чем я ожидал.
Because оценка is a feminine noun in Russian. Possessive pronouns agree with the noun they modify in gender, number, and case:
- моя оценка (feminine, nominative, singular)
- Compare: мой балл (masculine), моё письмо (neuter), мои оценки (plural)
It’s nominative singular because it is the subject of the sentence: Моя оценка … ниже.
по + dative is a very common way to mean “in (a subject/field), for (a class), on (a topic)” in contexts like grades, homework, and school subjects.
- оценка по математике = a grade for/in math
- Other examples: домашнее задание по русскому, экзамен по истории
Here математике is dative (see next question).