这段文字中的语法错误如下：

"this is a graph, about what do students use moblie phones to do."
错误：

“about what do”这个结构不对，应该是“about what students use mobile phones to do.”
"moblie"拼写错误，应该是"mobile"。
修改后：

"This is a graph about what students use mobile phones to do."

"as plain as the nose on your face, twenty-one percent of students use mobile phones to play games, fifteen percent to listen to music, thrity-three percent to check social media, five percent and eight percent for other and shopping,and only percent of students ues mobile phones to study."
错误：

“thrity-three”拼写错误，应该是“thirty-three”。
“five percent and eight percent for other and shopping”这部分不太清楚，应该是“five percent for other things and eight percent for shopping”。
“only percent”少了数字，应该填写具体的百分比。
“ues”拼写错误，应该是“use”。
修改后：

"As plain as the nose on your face, twenty-one percent of students use mobile phones to play games, fifteen percent to listen to music, thirty-three percent to check social media, five percent for other things, and eight percent for shopping, and only a small percentage of students use mobile phones to study."

"we can understand by those, the majority of students used mobile phones at entertainment."
错误：

“by those”这个表达不太准确，应该改为“from this”。
"used"应该改为“use”来保持时态一致。
"at entertainment"不太自然，应该改为“for entertainment”。
修改后：

"We can understand from this that the majority of students use mobile phones for entertainment."

"ideed, use phone to entertainment is good way to relax, but if you spend much time to playgame, rather than improve oneself by phone. you will fell no gain when you finish entertainment time."
错误：

“ideed”拼写错误，应该是“Indeed”。
“use phone to entertainment”不太自然，应该改为“using a phone for entertainment”。
"good way to relax"前面应该加上“a”，变成“a good way to relax”。
“spend much time to playgame”应该改为“spend much time playing games”。
"rather than improve oneself by phone"应该改为“rather than improving oneself with a phone”。
"fell"应该改为“feel”。
修改后：

"Indeed, using a phone for entertainment is a good way to relax, but if you spend much time playing games, rather than improving yourself with a phone, you will feel no gain when you finish entertainment time."

"but on the contray, if you spend the majority time to study, when it is done, you will feel like a rewarding experience."
错误：

“contray”拼写错误，应该是“contrary”。
“spend the majority time to study”应该改为“spend the majority of your time studying”。
“when it is done”应该改为“when you are done”。
“feel like a rewarding experience”表达不太清晰，应该改为“feel that it was a rewarding experience”。
修改后：

"But on the contrary, if you spend the majority of your time studying, when you are done, you will feel that it was a rewarding experience."

修改后的完整段落：

This is a graph about what students use mobile phones to do. As plain as the nose on your face, twenty-one percent of students use mobile phones to play games, fifteen percent to listen to music, thirty-three percent to check social media, five percent for other things, and eight percent for shopping, and only a small percentage of students use mobile phones to study.

We can understand from this that the majority of students use mobile phones for entertainment. Indeed, using a phone for entertainment is a good way to relax, but if you spend much time playing games, rather than improving yourself with a phone, you will feel no gain when you finish entertainment time. But on the contrary, if you spend the majority of your time studying, when you are done, you will feel that it was a rewarding experience.