Breakdown of Завтра мы будем гулять с твоим старшим братом.
брат
brother
мы
we
твой
your [informal]
быть
to be
Questions & Answers about Завтра мы будем гулять с твоим старшим братом.
Why do the words твоим старшим братом have these endings?
They are in the Instrumental case. The preposition с means "with" (doing something together with someone) and requires the Instrumental case for the noun and any adjectives or pronouns describing it. Here, the dictionary forms твой старший брат change to твоим старшим братом.
Why is the instrumental ending for старшим spelled with -им instead of -ым?
This is due to a spelling rule. Normally, hard masculine adjectives in the Instrumental case end in -ым (like новым). However, after the "hushing" letter ш, Russian spelling rules forbid the letter ы. Therefore, we must use и, making the ending -им.
What is the dictionary form of твоим?
The dictionary form is твой (your/yours for masculine nouns). In the Instrumental case, masculine possessive pronouns take the ending -им, so твой becomes твоим. Notice how it rhymes with the -им ending of the adjective старшим right next to it.
Why does the sentence use будем гулять instead of a single-word future tense verb?