Learn Finnish: Herra ministeri on jo täällä.

Both words mean "here", but they have a different scope. Tässä refers to an exact, specific spot (like right here on this chair). Täällä refers to a broader area (like in this building or at this event). Since the minister has arrived at the general venue, täällä is the natural choice.

How does the use of herra differ from the English "Mr."?

In Finnish, titles are much less common in everyday life. When herra is used, it is typically paired with a professional or political title in formal situations, like herra ministeri (Mr. Minister) or herra puheenjohtaja (Mr. Chairman). You rarely hear it used with just a last name in modern Finnish unless the situation is extremely formal.

Since this lesson focuses on the comitative case, how would we add "with his spouse" to this sentence using that case?

You would use the word puoliso (spouse) and add the comitative ending -ine plus the third-person possessive suffix -en, making it puolisoineen. The full sentence becomes Herra ministeri on jo täällä puolisoineen. Remember that comitative words always take a plural-looking marker even if referring to a single person, and they almost always require a possessive suffix.