This is a watermelon.
It is a watermelon.由於西瓜是可數名詞,所以西瓜的前面一定要有個a去修飾。這是用英文表達是:this is或者it is。如果這個句子要改寫為複數句子,那麼就是These are watermelons.
I like watermelons.I like eating watermelons in summer.