Is the sentence correct?
Yes — your sentence is grammatically correct. It starts with a capital letter and ends with a period. Here it is again:
A sailboat moves when the wind pushes its sails.
Other clear ways to say the same thing:
- A sailboat moves when the wind pushes against its sails.
- A sailboat moves when the wind fills its sails.
What this means (easy steps for a 7-year-old):
- Wind blows.
- The sails catch the wind.
- The wind pushes the sails, and the sails push the boat.
- The boat moves through the water.
Your original sentence is simple and correct — great job!