Learn Swedish: Du måste plantera fler träd på gården om du vill skydda miljön. - You must plant more trees on the farm if you want to protect the environment.

It is because the main clause comes first! Since Du måste plantera... starts the sentence, it follows normal Subject-Verb order. If the conditional clause came first (Om du vill skydda miljön...), the main clause would have to invert its subject and verb to satisfy the V2 (verb-second) rule: ...måste du plantera fler träd på gården.

Why use fler instead of mer for "more"?

English uses "more" for everything, but Swedish makes a distinction. You use fler for things you can count (like trees, dogs, or chairs) and mer for uncountable amounts (like water, coffee, or time). Since trees are countable, we use fler.

Is träd plural here? It looks exactly like the singular form.

Yes, it is plural! Träd is an ett-word that ends in a consonant (ett träd). Words in this specific group do not change their form in the indefinite plural. We know it means "trees" rather than "tree" here because it is preceded by the word fler (more).

Why isn't there an att (to) before plantera or skydda?