Learn Hardcore Russian: Утром туман рассеется.

In Russian, certain time-of-day expressions—утро (morning), день (day/afternoon), вечер (evening) and ночь (night)—take the instrumental case to function like adverbs (e.g. утром, днём, вечером, ночью) without any preposition. This construction answers “when?” without an extra word like в.

Why isn’t there a preposition before утром (for example, в утро)?

Time-of-day nouns in the instrumental case stand alone as adverbials in Russian. In these fixed expressions you never add в, so you say утром (“in the morning”), not в утро.

What case is туман, and why is it used here?

Туман is in the nominative case because it is the subject of the sentence—the entity performing the action of clearing. In Russian, subjects are almost always in the nominative.

Why is there no separate word for “will” to express the future tense?