Learn Russian: Если фермер много работает, то урожай будет большой. - If the farmer works a lot, then the harvest will be big.

The farmer is working right now or generally (present), which will cause the harvest to be big later (future). Note that if the work was entirely in the future, Russian allows you to use the future tense in the 'if' part too: Если фермер будет много работать... This is different from English, which forces the present tense for future 'if' clauses.

Is the word то required to say 'then'?

It is optional but very common. то acts as 'then' in an 'if... then...' structure. You can leave it out and simply use a comma to separate the two ideas, just like in English: Если фермер много работает, урожай будет большой.

Why does много come before the verb instead of after it like in English?

Russian word order is flexible, but it is very natural to place adverbs like много (a lot) right before the verb they describe. You will hear both много работает and работает много, but putting it before the verb is standard and sounds very smooth.

Why isn't большой in the instrumental case? Doesn't будет usually require it?

While it is true that будет (will be) often takes the instrumental case for professions or temporary states (like 'he will be a doctor'), simple adjectives describing a subject often just stay in the nominative case. Saying будет большой is perfectly correct and natural here.