Learn Polish: To wydaje się trudne.

The word się changes the meaning of the verb. On its own, wydawać means to spend money or to hand something over. But as a reflexive verb, wydawać się, it means to seem or to appear.

Why is the adjective trudne used instead of the adverb trudno?

In this sentence, To (that/this) is the specific subject, and it acts as a neuter noun. When you describe a subject with an adjective, they must match in gender. So we use the neuter adjective trudne. If the sentence had no specific subject (for example, saying 'It is difficult to do'), you would use the adverb trudno instead.

Could I change the word order and put się at the very end of the sentence?

It is highly discouraged to put się at the very end of a sentence if you have other options, and it can never go at the very beginning. Placing it right after the verb, as in To wydaje się trudne, is the most natural word order here.

I have learned that some linking verbs make the following word take the Instrumental case. Why is trudne in the basic Nominative case here?

When a verb links a subject to a standalone adjective describing it, the adjective usually stays in the basic Nominative case to match the subject. The Instrumental case is mostly used when you link the subject to a noun, rather than just an adjective.