Моё сомнение исчезает, когда учитель спокойно объясняет правило.

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Breakdown of Моё сомнение исчезает, когда учитель спокойно объясняет правило.

• учитель: the teacher
• когда: when
• правило: the rule

Questions & Answers about Моё сомнение исчезает, когда учитель спокойно объясняет правило.

Why is it моё сомнение, not моя сомнение or мой сомнение?

In Russian, possessive pronouns agree with the gender, number, and case of the noun they modify.

• сомнение is a neuter noun (it ends in -ие and its dictionary form is neuter).
• The neuter form of мой is моё.

So you must say:

• мой учитель (masculine)
• моя книга (feminine)
• моё сомнение (neuter)
• мои сомнения (plural)

Using моя or мой with сомнение would be grammatically wrong because of gender disagreement.

Why is сомнение singular? In English I might say “my doubts disappear.”

Russian often uses a singular abstract noun where English prefers a plural.

Моё сомнение исчезает can mean:

• My doubt disappears (about one specific thing), or
• My doubt disappears in a general sense (doubt as a state).

You could say Мои сомнения исчезают (my doubts disappear) if you want to emphasize multiple separate doubts. The singular just focuses on the overall feeling of doubt as one state.

Can I say Мои сомнения исчезают, когда… instead? Is that correct?

Yes, that is grammatically correct: