Learn Finnish: Isä rakensi eilen punaisen puutalon. - Dad built a red wooden house yesterday.

When the first part of a Finnish compound word describes the material something is made of, it stays in the basic nominative case. Therefore, puu (wood) + talo (house) correctly forms puutalo. If you used the genitive and wrote puuntalo, it would imply possession, literally meaning "the tree's house."

Since puutalon ends in -n, does that mean the whole compound word is in the genitive case here?

It looks identical to the genitive, but structurally it is the singular accusative case (often called the "total object" case). Because the action is completed—Dad built a whole house—the direct object takes the -n ending.

Why did the adjective punainen change to punaisen?

In Finnish, an adjective must agree in case with the noun it modifies. Since the object of the sentence (puutalon) is in the accusative case ending in -n, the adjective punainen (red) must also take the -n ending, changing its stem to punaise- and becoming punaisen.

Is it normal to put the time word eilen (yesterday) right after the verb? In English, it usually goes at the end.