Learn Finnish: He hiiviskelivät pimeässä meidän huomaamattamme. - They sneaked around in the dark without us noticing.

It comes from the verb huomata (to notice). First, we form the 3rd infinitive abessive by adding -matta (meaning without doing) to the strong stem, giving us huomaamatta (without noticing). Then, to specify who didn't notice, we add the first-person plural possessive suffix -mme, resulting in huomaamattamme.

Why do we use the genitive pronoun meidän before huomaamattamme?

In this structure, the subject of the unperformed action (the ones who are not noticing) is put into the genitive case, which is meidän. It works exactly like a regular possessive structure, but instead of owning a noun, we grammatically "own" the action of not noticing.

Could we leave out meidän and just say He hiiviskelivät pimeässä huomaamattamme?

Yes, you can! Just like with regular possession, the first and second person pronouns (minun, sinun, meidän, teidän) can be dropped because the possessive suffix (-mme) already clearly tells us who it refers to. However, third-person pronouns (hänen, heidän) would have to be kept if they were the ones not noticing.

I know the verb hiipiä means "to sneak". What does the -skellä ending in hiiviskelivät add?