Learn Polish: Sprawdzę to z bratem. - I will check this with my brother.

This is the magic of perfective verbs! Sprawdzić is a perfective verb, meaning it describes a completed, one-time action. In Polish, when you add a present-tense ending (like -ę for "I") to a perfective verb stem, it automatically creates the future tense. There is no present tense for completed actions!

Where is the word for "I" in this sentence?

In Polish, the subject pronoun ja (I) is usually dropped because the verb ending already tells us who is doing the action. The -ę at the end of sprawdzę clearly means "I will check." You would only add ja if you wanted to emphasize that you are doing it, not someone else.

Why does the word for brother change to bratem here?

The basic word for "brother" is brat. However, the preposition z (meaning "with") requires the noun following it to take the Instrumental case. For most masculine nouns, this means adding the ending -em. So, z bratem means "with a brother" or "with my brother."

The English translation says "with my brother", but I don't see a word for "my" in the Polish sentence. Why?

Polish often drops possessive words like "my" or "your" when it is obvious from the context. Since the speaker is the one doing the checking, it is naturally assumed they mean their own brother. If you really wanted to specify, you could say z moim bratem, but it is usually not necessary.