Learn Russian: Если фермер посадит зерно, то он будет богатый. - If the farmer plants the grain, then he will be rich.

In Russian, if a condition refers to the future, you must use the future tense in the "if" clause. English uses the present tense for this ("If it rains...", "If he plants..."), but Russian is more literal. Since the planting hasn't happened yet, it requires the future tense посадит (will plant).

What is the word то doing here? Do I always need it?

то translates to "then" and pairs with Если (If) to form an Если ..., то ... (If ..., then ...) structure. It is technically optional, so you could drop it and just use a comma, but including то is very common and makes the cause-and-effect relationship sound stronger.

Why use the perfective verb посадит instead of an imperfective verb?

The perfective aspect shows that the action will be completely finished. The farmer needs to successfully finish planting the grain to get the result (becoming rich). An imperfective verb would just mean he spends time in the process of planting, without guaranteeing the action gets completed.

Is the word зерно (grain) in the nominative or accusative case here?

It is in the accusative case because it is the direct object—the thing being planted by the farmer. However, because зерно is an inanimate neuter noun, its accusative form looks exactly the same as its dictionary (nominative) form.