Questions & Answers about Его ответ был убедительным.
Why is его used here? Does it mean his or him?
Here его means his because it directly modifies the noun ответ.
The same form его can also mean him in other sentences, but here the structure makes it clearly possessive:
- его ответ = his answer
A useful thing to remember: Russian third-person possessives его, её, and их do not change to match the noun they go with:
- его ответ
- его книга
- его письма
All of those still use его.
Why is ответ in the form ответ, not ответа or another case?
Ответ is the subject of the sentence, so it is in the nominative singular.
The sentence is about the answer, and that is the thing described as convincing. So the basic structure is:
- ответ = subject
- был = was
- убедительным = convincing
The dictionary form is also ответ.
Why is the verb был masculine?