Learn Finnish: Mä auttais vähävaraisia, jos mulla ois parempi toimeentulo. - I would help the underprivileged if I had a better income.

Vähävaraisia is the adjective vähävarainen (underprivileged or low-income) acting independently as a noun to mean the demographic group "the underprivileged". It is in the plural because it refers to a group of people. It takes the partitive case because the verb auttaa (to help) always requires its object to be in the partitive.

I recognize the conditional marker in auttais, but why is there no personal ending like -in?

In colloquial spoken Finnish, it is very common to drop the first-person singular ending -n in the conditional mood. So, the standard minä auttaisin becomes mä auttais. Because the spoken pronoun mä is explicitly used, the person doing the action remains perfectly clear.

What word is ois a shortened form of?

Ois is a very common spoken-language form of olisi, which is the conditional form of olla (to be). The phrase mulla ois is simply the spoken version of the "I have" structure in the conditional: minulla olisi (I would have).

How is the noun toimeentulo (income or livelihood) formed?