Learn Finnish: Tästä hetkestä lähtien se on elinikäinen tavoite. - From this moment on, it is a lifelong goal.

The postposition lähtien requires the noun phrase it modifies to be in the elative case. Because the elative often expresses movement out of or away from something, it pairs naturally with lähtien to mean starting from a specific point in time or space.

Could I use pitäen instead of lähtien in this sentence?

Yes, you could say Tästä hetkestä pitäen and the meaning remains exactly the same: from this moment on. Both postpositions require the elative case and indicate starting points. However, lähtien is generally more common in everyday spoken Finnish.

How is the adjective elinikäinen constructed?

It is a compound word based on the noun elinikä (lifetime). This comes from elin- (a prefix form related to living or vital functions) and ikä (age). By adding the common adjective suffix -inen, it becomes elinikäinen, meaning lifelong.

I know the word päämäärä also means goal. Could I use it here instead of tavoite?

You could, but they have a slightly different feel. A tavoite is typically a concrete, actionable objective or target. A päämäärä is more of an ultimate destination or overarching life purpose. Depending on what the pronoun se refers to, both can work well for a lifelong ambition.