Learn Finnish: Avain on maton alla. - The key is under the rug.

The word alla (under) is a postposition. In Finnish, most postpositions require the noun that comes right before them to take the genitive (possessive) -n ending. It can help to think of the literal translation as "at the rug's underside" (maton alla).

The basic dictionary word for rug is matto. Why does maton only have one "t"?

This happens because of a spelling rule called consonant gradation. When you add the genitive -n ending, the final syllable of the word becomes "closed" (it ends in a consonant instead of a vowel). This forces the strong double consonant tt to weaken into a single t.

In English, "under" comes before the noun. Can I swap the order and say alla maton?

No, you must say maton alla. Because alla is a postposition, it has to come after the noun it refers to. While English uses prepositions (placed before the noun), Finnish relies heavily on postpositions (placed after the noun).

How do we know it translates to "The key" and not "A key" if there are no words for "the" or "a" in the sentence?

Finnish doesn't have articles like "a" or "the". You usually figure it out from context and word order. When a noun like avain (key) is placed at the very beginning of a sentence to state its location, it is treated as specific, known information, which translates best as "the key" in English.