What is the subject of the following sentence?: The red bicycle is at the repair shop, since it has a flat tire.
The subject is 'bicycle'.
The subject of a centence is a noun (proper or general) for which you can ask a question beginning with Who? (for people) or What? (for things).
In this sentence you can ask a question: 'What is at the repair shop?'. The answer for this question would be the bicycle, so you can write that it is the subject of a sentence.