Correct sentence: In summer, mosquitoes fly in cloud-shaped swarms.
This sentence is grammatically correct. Here is why, in simple steps:
- In summer tells when something happens. A comma after an introductory phrase like this is fine but not always required.
- Mosquitoes is the subject (who). Fly is the verb (what they do). They match because both are plural.
- In cloud-shaped swarms tells how they fly. The words 'cloud-shaped' form an adjective (two words that together describe 'swarms'), so the hyphen is correct and makes the meaning clear.
Other good ways to say it: In the summer, mosquitoes fly in cloud-like swarms. or In summer, mosquitoes fly in swarms that look like clouds.
Nice sentence — good job!