Learn Russian: Завтра мы обойдём это глубокое озеро за пять часов. - Tomorrow we will walk around this deep lake in five hours.

The prefix об- changes to обо- to make pronunciation easier before certain root words, like идти (to walk). The infinitive is обойти (to walk around), and it conjugates as обойдём for the "we" form. Like most verbs with this prefix meaning "around", it requires a direct object in the Accusative case.

Why do we use the perfective verb обойдём instead of the imperfective будем обходить?

We use the perfective because the focus is on successfully completing the action (walking all the way around the whole lake). This pairs logically with the time phrase за пять часов, which states the exact time needed to finish a task.

What is the difference between saying за пять часов and just пять часов?

Adding the preposition за before an amount of time tells us how long it takes to achieve a finished result ("in five hours"). If you just said пять часов without за, it would mean "for five hours" (an ongoing process) and would strictly require the imperfective verb будем обходить.

The verb requires the Accusative case, so why doesn't the ending of это глубокое озеро change?