Learn Finnish: Tänne rakennettaisiin leveä pyörätie. - A wide bike path would be built here.

It comes from the verb rakentaa (to build). You start with the passive stem rakenneta- (seen in the present passive rakennetaan), add the conditional marker -isi-, and finish with the passive ending -in. Notice the consonant gradation: the strong -nt- in rakentaa becomes the weak -nn- in rakennettaisiin.

Why does the sentence use tänne (to here) instead of täällä (here)?

In Finnish, verbs like rakentaa (to build) are treated as directional. Because you are bringing a new bike path into existence at a destination, Finnish expresses this as building it into a place. Therefore, you must use the directional word tänne rather than the static location word täällä.

Since the bike path is the object being built, why is leveä pyörätie in the basic nominative form?

In passive sentences, a total object (an object that would normally take the -n ending in an active sentence) is put into the basic nominative case instead. If the sentence were active, like 'He would build a wide bike path', the object would take the -n ending (leveän pyörätien).

Could rakennettaisiin also translate to 'we would build'?