Learn German: Ich finde die sanfte Massage wunderbar. - I find the gentle massage wonderful.

This is an example of an adjective ending after a definite article. Massage is a feminine noun (die Massage), and in this sentence it is the direct object (accusative case). After the feminine article die, the adjective takes a simple -e ending.

Why doesn't wunderbar have an ending, since it also describes the massage?

In German, adjectives only take endings when they are placed directly in front of a noun (like die sanfte Massage). When an adjective comes later in the sentence to describe how something is (a predicate adjective), it always stays in its basic dictionary form without any endings.

Can I use finden to express my opinion about anything?

Yes! While finden literally means to physically find something, it is very commonly used exactly like this to express an opinion. It works just like saying "I find it..." or "I think..." in English.