Learn Polish: Mój ojciec uczy dzieci żeglarstwa. - My father teaches children sailing.

This sentence demonstrates the double-object rule for the verb uczyć (to teach). The person receiving the teaching is in the Accusative case (dzieci), while the subject being taught is in the Genitive case (żeglarstwa).

Dzieci is the Accusative object here, but it doesn't look like a standard Accusative plural. Why is that?

The word for child (dziecko) has a highly irregular plural. Its Nominative, Accusative, and Genitive plural forms are all identical: dzieci. You have to rely on context and the verb's double-object requirements to know it is Accusative in this sentence.

What case is żeglarstwa and what is its base form?

It is in the Genitive case, as required for the subject being taught. The base dictionary form is the neuter noun żeglarstwo (sailing). It uses the common suffix -stwo, which is often added to professions (like żeglarz, meaning sailor) to create abstract nouns representing the activity, field, or collective group.

If we changed the verb to its perfective form, nauczy, how would it change the sentence?

Because nauczy is perfective, it cannot describe an ongoing present action. The sentence Mój ojciec nauczy dzieci żeglarstwa would shift to the future tense, meaning 'My father will teach the children sailing,' with a focus on the successful completion of learning the skill.