前两个中文句子很好！
 
3 如果我们知道直角三角形的直角边a和b，那么这项三角形的斜边c，就可以由关系式 a*a + b*b = c*c 求出来。 
 

First I want to say your Chinese are so good. There are mistakes about, but It doesn't prevent others from understanding.

请改一下这三个句子 my English needs to be corrected too...

1 练习和不断重复是学习中有效的方法，特别对于语言学习来说,这两者都是很重要. 

Practicing and repeating are the efficient learning methods, especially in the progress of learning languages. Practice and repetition are playing an important role. 

2 在现代世界上，英语口语能力占有很重要的地位。 

In the world we are living on today, the ability of speaking English plays a decisive role. 

3 如果我们知道正交三角形的直角边a和b，那么这个三角形的斜边c，就可以由关系式 a*a + b*b = c*c 求出来。 

If we know the length of the legs a and b of a right-angled triangle, then we can use the equation a*a + b*b = c*c to calculate the value of the hypotenuse c.

请改一下这三个句子 my english needs to be correctet too...

1 练习和复习是学习的很有效的学习方法,特别是在学语言的过程中,练习和复习占有很重要的地位.

Practicing and repeating is an effecient learning method. Especially for the progress of learning languages, practice and repetition are playing an important role.

2 在现代的世界上，会说英语的能力占有很重要的地位。（有的时候不一定所以的单词都要一对一的翻译出来） 

In the world we are living on today, the ability for speaking english plays a decisive role.

3 如果我们知道直角三角形的直角边a和b，那么这个三角形的斜边c，就可以由关系式 a*a + b*b = c*c 求出来。

if we know the length of the legs a and b of a right-angled triangle, then we can use the equation a*a + b*b = c*c to calculate the value of the hypotenuse c.