"Once upon a time, there lived an old man and his wife." Events of the past like this one, no matter if fact or fairy tale, are normally reported using the past tense. This crucial grammatical distinction between what is and what was is available in most of the world's languages. Needless to say Japanese has it, too — and knows quite a lot to do with it.

At first look, it seems that the Japanese past tense is a pretty easy thing to handle. Thus it always ends in ta (granted, sometimes it's da), and doesn't have to compete with any other past forms. This spares the learner quite some interpretative hair-splitting. For example, when you didn't make it to a movie on time, you would say やっと映画館に着いた時、 映画はもう始まっていた (Yatto eigakan ni tsuita toki, eiga wa mō hajimatte-ita). Unlike in its English translation, "When I finally arrived at the cinema, the film had already started," the fact that the beginning of the film predated your arrival on scene isn't expressed by a pluperfect form ("had started"), but by the contrast between simple past (tsuita) and past progressive (hajimatte-ita).

So much for the easy part. The first shock to the unsuspecting language learner (this one included) comes when they realize that past forms are not restricted to verbs but are also available for adjectives. It took me quite some time to get over the fact that an enjoyable experience in the past is not reported as 楽しいでした (Tanoshii deshita) but 楽しかったです (Tanoshikatta desu), which — very literally — is not "It was pleasant" but "It's pleasant-ed."