Learn Finnish: Vahva usko auttaa vaikeana aikana. - Strong faith helps in a difficult time.

In Finnish, the essive case (-na/-nä) is the standard way to express "during" or "in" a certain period of time, especially with words like aika (time), päivä (day), or vuosi (year). Using the inessive here (ajassa) would sound unnatural, as it typically refers to physical space or specific idioms.

Is the noun usko just the stem of the verb uskoa?

Yes! Many abstract nouns in Finnish are formed simply by taking the verb stem. From uskoa (to believe) we get usko (faith/belief), just like toivoa (to hope) gives toivo (hope).

Since we are learning about abstract nouns that take a 1st infinitive, could I say usko selvitä (faith to survive)?

You should avoid that. While nouns like kyky (ability) or halu (desire) smoothly take a 1st infinitive, usko typically does not. Instead, usko usually takes a noun in the illative case (e.g., usko parempaan huomiseen, faith in a better tomorrow) or a clause with siihen, että... (belief in the fact that...).

If I wanted to specify who the faith helps (e.g., "helps us"), what case would the pronoun take?

The verb auttaa always requires its object to be in the partitive case. To say "helps us," you would add the partitive pronoun meitä: Vahva usko auttaa meitä vaikeana aikana.