In the following sentence, find the clause that is the main subject.
He was walking in the park when it started to rain.
The main subject of the sentence is "He." The main clause of the sentence is "He was walking in the park."
Complete the following task based on the instruction and optional input provided.### Instruction:
In the following sentence, find the clause that is the main subject.

### Input:
He was walking in the park when it started to rain.

### Output:
The main subject of the sentence is "He." The main clause of the sentence is "He was walking in the park."