Learn Finnish: Tämä on talo.

Finnish doesn't have articles like "a", "an", or "the". The word talo can mean "a house" or "the house" depending on the context. In this sentence, "This is a house" is the most natural translation.

When should I use tämä instead of se?

Tämä means "this", pointing to something physically close to the speaker. Se means "it", referring to something already mentioned or slightly further away. Because we are pointing out a house right in front of us, tämä is used here.

Why does tämä use the verb on?

Tämä (this) is a third-person singular word, just like "he", "she", or "it". Because of this, it uses on, which is the "he/she/it is" form of the verb olla (to be).

Why doesn't talo have any special endings added to it here?

When you are simply stating what something is by saying "This is a...", the noun stays in its basic dictionary form. In grammar, this basic form is called the nominative case. So, talo stays exactly as you would find it in a dictionary without any extra letters on the end.