As it has been already said, when we pronounce an utterance the voice goes up and down and the different notes of the voice combine to make tunes. In English the tune belongs not to a single word but to the whole utterance or word group. The shape of the tune is decided partly by the number of important words in an utterance and partly by the exact attitude you wish to express. Important words carry most of the meaning in an utterance. An important word always has a stressed syllable and usually has a change of pitch connected to it. But not all stressed words in an utterance can be called important: some words may be stressed simply to preserve rhythm, and the fact that there is no change of pitch shows that the speaker does not treat them as important.
Also the attitude of the speaker, his feelings as he says an utterance, affects the tune-shape, and affects it very much, as we shall see later.