Correct sentence: In autumn, when it rains, the forest is full of mushrooms.
Why this is better (easy words):strong>
- "When it rains" sounds more natural than "when it is rainy."
- We use commas before and after "when it rains" because this little part gives extra information. If you take it away, the sentence still makes sense: "In autumn the forest is full of mushrooms."
Another good version you can say: In autumn, when it's rainy, the forest is full of mushrooms.
Try this: Say the sentence out loud. If you pause a little around the middle part (the rain part), keep the commas.