Learn Finnish: Järvi jäätyy vähittäin. - The lake freezes gradually.

The suffix -ittain/-ittäin often translates to "by" or "in groups of". It is formed from the base word vähä (little). Adding this suffix creates the literal meaning "little by little", which we translate naturally to "gradually".

What happens to the end of the word vähä before adding the suffix?

The distributive suffix -ittain/-ittäin always attaches to the plural stem of a word. For a word ending in -ä like vähä, the plural marker -i- replaces the final vowel. This creates the plural stem vähi-, to which we then add -ttäin.

Why is the suffix spelled -ittäin here instead of -ittain?

This is due to vowel harmony. Because the root word vähä contains the front vowel ä, the suffix must also use front vowels. Therefore, we use -ittäin with ä rather than -ittain with a.

Why is the verb jäätyy used here instead of jäädyttää?

The verb jäätyä is an intransitive verb, meaning the subject undergoes the action itself without taking an outside object (the lake freezes on its own). If a person or a machine was actively freezing something, you would instead use the transitive verb jäädyttää.