Learn Finnish: Talon takana haisee pahalta. - It smells bad behind the house.

Verbs of perception, like haista (to smell), require the adjective describing the sense to be in the ablative case (-lta/-ltä). Because of this grammar rule, paha becomes pahalta.

The lesson lists tuoksua for "to smell". Why does this sentence use haisee?

Both verbs mean "to smell", but they have different feelings. Tuoksua is used for pleasant or neutral smells (like flowers or fresh bread). Haista is specifically used for bad or unpleasant smells. Since the smell here is bad (pahalta), haista is the natural choice.

Why is there no word for "It" at the beginning of the Finnish sentence?

In English, we need "it" as a dummy subject for general statements like "it rains" or "it smells". Finnish doesn't use a pronoun like se (it) for these. You just use the verb in the third-person singular (haisee) and let it stand alone.

Why is "house" in the genitive case (talon) here?

The word takana (behind) is a postposition. In Finnish, the noun that comes right before a postposition almost always has to be in the genitive case, which ends in -n. Because of this, talo becomes talon.