Learn Finnish: Äidin ikä on neljäkymmentä vuotta.

The base word is äiti (mother). To say "mother's" (possession), we add the genitive ending -n. Adding this ending closes the syllable, which causes a spelling change called consonant gradation. The hard consonant t softens to a d, giving us äidin.

Does the possessor word always come before the thing it owns?

Yes! Just like in English when we say "mother's age", the word with the genitive -n ending (äidin) comes right before the thing being owned (ikä).

Why is the word for "years" vuotta instead of the dictionary word vuosi?

In Finnish, any number greater than one must be followed by a noun in the partitive case. vuotta is the partitive form of vuosi (year). This is a very common rule you will use every time you count things!

How is the word for forty (neljäkymmentä) built?

It literally translates to "four tens". It is made by combining neljä (four) and kymmentä, which is the partitive form of kymmenen (ten). It uses the partitive for exactly the same reason as vuotta—because it comes after the number four!