Learn Finnish: Nainen on vaatekaupassa sovittamassa uutta paitaa. - The woman is in the clothing store trying on a new shirt.

It comes from the verb sovittaa (to try on clothes). To make this form, you take the strong stem sovitta-, add -ma- (the third infinitive marker), and then add the inessive ending -ssa (meaning "in"). Together, sovittamassa literally means "in the act of trying on".

Why do we use on ... sovittamassa instead of just saying she tries on a shirt?

Using the verb olla (to be) with the -massa form emphasizes that an action is currently ongoing, usually at a specific location. It acts a lot like the English continuous tense ("is doing"). Here, it highlights that she is actively in the middle of trying on the shirt right now at the store.

Why are the words for "new shirt" in the partitive case (uutta paitaa)?

The verb sovittaa (to try on) takes a partitive object. In addition to this, because the sentence describes an ongoing, incomplete action (she is in the middle of trying it on), the object must be in the partitive case. The word uusi becomes uutta and paita becomes paitaa.

Why does the word vaatekaupassa have only one "p" when the word for store is kauppa?