Questions & Answers about Нам нужно обойти яму и стройку.
Why is the prefix обо- used instead of об- in обойти?
The prefix об- often gains an extra -о before roots starting with certain consonant clusters or sounds to make it easier to pronounce. When combining the prefix with the base verb идти (to go), it forms обойти rather than the unpronounceable 'обйти'.
Why are яму and стройку in the Accusative case? Doesn't 'around' usually require a preposition?
While the standalone preposition вокруг (around) requires the Genitive case, verbs of motion with the об-/обо- prefix take a direct object in the Accusative case. The 'around' or 'bypassing' meaning is already built right into the verb обойти, so you do not need an extra preposition here.
Why use the perfective обойти here instead of the imperfective обходить?
We use the perfective обойти because this refers to a specific, single action that needs to be successfully completed (bypassing this specific pit and construction site right now). If it were a regular habit, like 'We always need to go around the construction site', you would use the imperfective обходить.
Just as a refresher, why does the sentence start with Нам and not Мы?
The word нужно (necessary/needed) requires the person experiencing the need to be in the Dative case. The sentence literally structures this as 'To us it is necessary to go around the pit and the construction site.'