The components of this sentence are:
personal pronoun, first person, singular, nominative case
verb (a form of "sprechen"), first person, singular, present tense
mit meinem Bruder
The verb sprechen (to speak, to talk) can have these kinds of objects:
What are you speaking? (What is coming out of your mouth when you are speaking?) (Rare: Who are you speaking?)
Ich spreche deine Sprache. (I speak your language.)
Ich spreche nur den ersten Satz. (I speak only the first sentence.)
Ich spreche meinen Bruder.
The last sentence is rare and can have two meanings:
- I will meet my brother and will have a conversation with him. You use this version often with an additional temporal adverb (»Morgen spreche ich meinen Bruder und da werden wir dann die weitere Vorgehensweise klären.« = Tomorrow I will meet my brother and then we will clarify the further course of action.) (This example also shows, that German Präsens can be future tense in English.)
- There is an audio drama about my brother, and in this drama I am speaking the role of my brother.
prepositional object with "über"
About who or what are you speaking? (What is the topic of the conversation?)
Ich spreche über das Wetter. (I talk about the weather.)
Ich spreche über das Auto. (I talk about the car.)
Ich spreche über meinen Bruder. (I talk about my brother.)
prepositional object with "mit"
With whom are you talking? (Who is the other participant of the conversation?)
Ich spreche mit dem Lehrer. (I talk with the teacher.)
Ich spreche mit der Chefin. (I talk with the boss.)
Ich spreche mit meinem Bruder. (I talk with my brother.)
Note, that there are also prepositional objects with other predicates and that you also can have more of them in a sentence, like in this example (I put each prepositional object in square brackets):
»Der König spricht [während des Festes] [dank der neuen Technik] [durch ein Megaphon] [zu seinem Volk].« = The king speaks [during the festival] [thanks to the new technique] [through a megaphone] [to his people].
Prepositional objects consists of two parts. The first part is the preposition, and this preposition dictates the grammatical case of the second part:
- mit (with)
always needs dative case
Ich tanze mit meiner Tante (I dance with my aunt.)
Er hat sie mit einem scharfen Messer erstochen. (He stabbed her with a sharp knife.)
Er übt mit großer Leidenschaft. (He practices with great passion.)
These are some frequently used prepositions that always need dative:
aus, bei, mit, nach, seit
But there are also prepositions that always need accusative:
durch, für, gegen, ohne, um
Lots of prepositios need genitive case:
dank, trotz, fern, nördlich, während
And there are also prepositions which can go with dative or accusative, depending on the meaning:
auf, in, über, vor, zwischen
(Non of the lists is complete.)