In written form the expression of something that somebody else has said is quite easy.
John said: "The train arrives at seven in the morning."

If we want to repeat what John had said by telling somebody, we can't do this in the same way. We have to transform the phrases according to the following:
John said that the train arrived at seven in the morning.

The repetition of something somebody else has said is called reported speech. In English the system of the reported speech is very well defined and used within the limits of its definition. You've seen in the sentences above that the verbs are changed in their tenses (arrives => arrived). Let's have a look at the German version:

She said: "I'm eating a cake." Sie sagte: "Ich esse einen Kuchen." She said that she was eating a cake. Sie sagte, dass sie einen Kuchen esse.

You see the expression of the reported speech is in conjunctive (indicative: sie isst; conjunctive: sie esse). The tense (present) was not changed. So far so good. But thanks to the difficulties with the formation of the conjunctive things are not as easy as they seem to be.