Learn German: Wir sprechen mit dem Herrn. - We are speaking with the gentleman.

This is because Herr (gentleman/Mr.) belongs to a special group of masculine nouns called weak nouns (or N-declension). These nouns add an -n or -en ending in every case except the nominative (when they are the subject). Since it is the object of a preposition here, it becomes Herrn.

Why is it dem Herrn instead of den Herrn or der Herr?

The preposition mit (with) always forces the noun that follows it into the dative case. Because Herr is a masculine noun, its article changes from the nominative der to the dative dem.

Wait, doesn't the -n ending mean it's plural? How do I say 'with the gentlemen'?

It looks exactly like a plural, which is a very common trap! Because it is a weak noun, the singular adds just an -n (dem Herrn). The actual plural form adds -en. To say 'with the gentlemen' (plural dative), you would say mit den Herren.

Do you always use mit when talking about speaking to someone?

Yes! In English you might say 'speak to' or 'speak with', but in German, you use sprechen mit + dative when referring to the person you are conversing with.