Learn Finnish: Tämä nelikko laulaa ongelmitta. - This quartet sings without problems.

The suffix -kko / -kkö is used to create nouns that represent a group or cluster of things. Here, it is added to the number neljä (four) to mean a group of four, or a quartet. Notice the stem change: the ending -ä changes to -i-, giving us nelikko instead of neljäkkö.

Since a quartet has four people, why is the verb laulaa in the singular rather than the plural (laulavat)?

In Finnish, collective nouns, which are words that refer to a group but are grammatically singular, take a singular verb. Even though a nelikko consists of four people, the word itself is singular, so it takes the third-person singular laulaa.

What grammar case is being used in the word ongelmitta?

This is the abessive case, which indicates the lack or absence of something. It is formed by adding the suffix -tta / -ttä to the stem. Here, it is attached to the plural stem of ongelma (ongelmi-), translating directly to without problems.

Is there another way to say without problems besides ongelmitta?

Yes, you could use the preposition ilman followed by the partitive case: ilman ongelmia. Both mean exactly the same thing. The abessive case (ongelmitta) is slightly more concise and sounds very natural and idiomatic in Finnish, especially in written language.