Learn Finnish: Tämä kirja ei ole vaikea. - This book is not difficult.

In Finnish, negation works like a verb. ei is the "he/she/it" form of the negative verb, matching the book. To say "is not", you pair ei with ole. The word ole is a special base form of the verb olla (to be) that is used specifically in negative sentences.

Does the word ole change if we talk about a different subject, like "I am not"?

No, the word ole stays exactly the same! Only the negative verb changes to match the person. For example, "I am not" is en ole, "you are not" is et ole, and "this is not" is ei ole. The word ole is locked in this form.

Why is the adjective vaikea in its basic form here instead of the partitive case (vaikeaa)?

Learners often hear that negative sentences need the partitive case, but that rule applies to the object of an action (like what you are reading or eating). Here, vaikea is just describing the subject. Because a book (kirja) is a single, countable object, the adjective describing it stays in its basic form (the nominative case), even in a negative sentence.

Could I leave out the word kirja and just say Tämä ei ole vaikea?