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同学你好~
In the spring,we plant tomato seeds in the field and take care of them carefully, we can water them when they are necessary. 第二个逗号要改成句号。
when it is necessary这样表达，表示有必要的时候

In the summer,little tomatoes begin to grow,but they are smaller and green not to be eat.  
they are small, green, and inedible 不可食用的

They are gradually turn bigger and bigger, reder and reder.
turn是谓语，are多余，They gradually turn bigger and redder. 注意red的比较级

It is said that the tomatoes are benefit to people’s health.  
beneficial 形容词

祝同学学习进步~