Form is the style in which a a piece of text is written in, for example: novel, short story, play, dramatic monologue and sonnet. What is also included in the form of a text is the genre it is written in, examples can include gothic or comedy.
Structure is how the plot, or even simply sentences are constructed, therefore things to consider include: the chronology of the text, or from whose perspective the text is being written from.
Language is considered as the vocabulary used within a text, including language devices such as: pathetic fallacy, personification, direct speech and metaphor.