Learn Finnish: Täällä kasvaa pieniä sieniä.

When you talk about an undefined or unknown amount of plural things (like "some small mushrooms"), Finnish uses the partitive plural. If you used the regular plural pienet sienet, it would mean "the small mushrooms" referring to a specific, known group.

How do we get the ending -iä for words that already end in -i like pieni and sieni?

Words like pieni and sieni are old Finnish words, which means their grammar stems actually end in -e (piene-, siene-). To form the partitive plural, you add the plural marker -i- and the partitive ending -ä. The -e- drops out, leaving us with pieniä and sieniä.

Why is the verb kasvaa (grows) in the singular when the mushrooms are plural?

This is an "existential sentence" which describes something existing in a location. In Finnish, when a sentence starts with a location (täällä) and has a partitive subject (sieniä), the verb must always be in the third-person singular (kasvaa). You cannot use the plural verb form here!

Why use täällä for "here" instead of tässä?

Täällä refers to "here" in a general or wider area, like a forest or a yard. If you found a cluster of mushrooms and pointed right at them, you might use tässä, which means "right here" in a specific, exact spot.