Learn German: Der Hund war im Haus, aber er rennt heraus. - The dog was in the house, but he runs out.

her means motion towards the speaker, and aus means out of. By using heraus, the sentence implies the speaker is currently outside the house, watching the dog run out towards them. If the speaker were still inside the house watching the dog run away from them, they would use hinaus.

Why do we say im Haus instead of ins Haus?

im is short for in dem, which is the dative case. We use the dative case with two-way prepositions like in to describe a fixed location (answering "where?"). Since the dog was already inside, it is im Haus. If the dog were running into the house (a destination), we would use the accusative case: ins Haus.

Doesn't the verb go to the end of the sentence after a comma?

Not always! The word aber (but) is a coordinating conjunction. Words in this group (like und, oder, and aber) simply link two main sentences together without changing the word order. aber acts as a bridge and doesn't take up a numbered slot, so the subject er is position one, and the verb rennt stays in its normal second position.

Why is the dog referred to as er (he)? What if the dog is female?