Learn Finnish: Meidän täytyi hiiviskellä heidän tietämättään. - We had to sneak around without them knowing.

It is built using the 3rd infinitive abessive, which expresses 'without doing something'. We take the verb stem tietä- (from tietää), add the abessive ending -mättä (without), and finally attach the 3rd person possessive suffix -än to match the pronoun heidän.

Why do we need the word heidän here if the possessive suffix already indicates 'they'?

In this structure, the person who is not doing the action is expressed as a possessor. Because it is a 3rd person pronoun ('they'), standard Finnish grammar requires both the genitive pronoun heidän (their) and the matching possessive suffix -än on the verb form. If it were a 1st or 2nd person pronoun, you could drop the pronoun and just say, for example, tietämättäni (without my knowing).

I know the verb hiipiä means 'to sneak'. What is the difference between that and hiiviskellä?

Hiiviskellä is a frequentative verb derived from hiipiä. The -skellä ending implies that the action is continuous, repeated, or aimless. So while hiipiä is just 'to sneak' (usually from point A to point B), hiiviskellä translates better to 'sneaking around' or 'creeping about'.

Why is the verb täytyi used instead of a 'we' form like 'täydyimme'?