Breakdown of Он не возражает, но не совсем согласен с братьями.
брат
brother
не
not
он
he / it (for a masculine noun) / it (for a neuter noun)
но
but
Questions & Answers about Он не возражает, но не совсем согласен с братьями.
Is согласен a verb like "agree" is in English?
No, it is actually a short-form adjective! While English uses the verb "to agree", Russian uses the short adjective согласен (meaning "in agreement"). Because the subject is он (he), we use the masculine singular form.
Why do we use the preposition с and the Instrumental case for с братьями?
The short adjective согласен dictates the case here. It always requires the preposition с followed by the Instrumental case to show who you agree with, acting very much like "agree with" in English.
Why does the Instrumental plural for brothers, братьями, look a bit irregular?
The noun брат (brother) has an irregular plural stem: братья. Instead of standard noun endings, it takes special soft endings in the plural. To form the Instrumental plural, we add -ми to that stem, giving us братьями.
How does the word совсем change the phrase не согласен?
The adverb совсем means completely or entirely. Saying не совсем согласен softens the sentence to mean he does not entirely agree. If we dropped it and just said не согласен, it would mean he completely disagrees.