Learn Finnish: Kevyin varuste on myös kallein. - The lightest equipment is also the most expensive.

To make the superlative (the 'most' or '-est' form), you add the suffix -in to the word's stem. For words ending in -yt like kevyt (light), you drop the final -t to get the stem kevy-, and then add -in to make kevyin.

Why is the superlative of kallis spelled kallein instead of 'kalliin'?

The stem of kallis (expensive) is kallii-. When you add the superlative suffix -in to a stem that ends in a long vowel like -ii, Finnish changes the vowels to -ei- to make it easier to pronounce. That is why kallii- plus -in becomes kallein.

Why is varuste used in the singular here, when 'equipment' is usually an uncountable mass noun in English?

In Finnish, varuste refers to a single piece of equipment or gear, making it a normal countable noun. If you wanted to talk about multiple pieces of gear, you would use the plural varusteet, and the whole sentence would change to plural: Kevyimmät varusteet ovat myös kalleimmat.

Could I put myös at the very end of the sentence, like we do with 'too' in English?