Learn Finnish: Nähdäkseni tämä on hyvin totuudenmukainen sukutarina. - As I see it, this is a very truthful family story.

It is formed from the first infinitive nähdä (to see) plus the special translative marker -kse- and the first-person singular possessive suffix -ni (my). While it translates literally to something like 'for my seeing', it functions as a fixed adverb meaning 'as I see it' or 'in my view'.

If I wanted to say 'as WE see it', how would nähdäkseni change?

Because the end of nähdäkseni is just a standard possessive suffix, you simply swap it for the 'we' suffix -mme. The word becomes nähdäksemme.

How does the long adjective totuudenmukainen break down?

It is a compound made of totuuden (the genitive form of totuus, meaning 'truth') and mukainen (an adjective meaning 'conforming to' or 'in accordance with'). Together, it literally means 'in accordance with the truth', which translates to 'truthful' or 'true to life'.

Could I use tosi instead of hyvin here to mean 'very'?

You could, but hyvin (which usually means 'well') is slightly more formal and fits better in written or standard spoken Finnish. It pairs naturally with a formal-sounding, descriptive compound like totuudenmukainen, whereas tosi might sound slightly too casual for this specific sentence.