Learn Finnish: Koira juoksee sängyn alle. - The dog runs under the bed.

In Finnish, words indicating location like alle are usually postpositions. Just like a preposition in English comes before a noun, a postposition comes after the noun it belongs to. So "under the bed" is literally "the bed under".

Why is the word for bed sängyn instead of its basic form sänky?

Most postpositions require the noun before them to take the genitive case, which usually means adding an -n to the end. You can think of it as saying "to the bed's underneath" in English.

Where did the "g" in sängyn come from? Isn't the basic word sänky?

This happens because of a Finnish spelling rule called consonant gradation. When you close a syllable by adding an ending like -n, the consonant combination nk softens into ng.

I have seen the word alla used for "under". Why is it alle in this sentence?

Finnish postpositions change their endings to show movement. You use alle when something is moving into a position (the dog is running to get under the bed). If the dog was already resting under the bed, you would use alla to show it is just staying there.