The Internet has become an important part of teenage life. Most of them use the Internet to get lots of knowledges and information, and to relax in their spare time.

参考译文：互联网已经成为青少年生活的重要组成部分。他们中的大多数人利用互联网获取大量的多方面的知识和信息，并在他们的业余时间进行放松。

以上“knowledges”表示“多方面具体的知识技能”是可数名词，有复数形式。因此，以上句子是正确的。

(1)knowledge 表示抽象上“知识；学问；学识 ”意思时，是不可数名词。

have knowledge of sth

She told Parliament she had no knowledge of the affair. 

她告诉议会她对此事并不知情。

She is eager for the quest for scientific knowledge. 

她渴望对科学知识的探索

(2)to one's knowlege/to the best of one's knowlege据…所知

Alec never carried a gun to my knowledge.

据我所知亚历克从来不带枪。

To the best of my knowledge, Gloria did not make these comments. 

据我所知，格洛丽亚没这么说过。

(3)safe in the knowledge that因知道…而放心

You can let your kids play here, safe in the knowledge that they won't get sunburn. 

你可以放心让孩子们在这儿玩，他们肯定不会晒黑。

备注：当表示具体的多方面的知识技能时，是可数名词，有复数knowledges

（当抽象名词具体化后就成了可数名词，是有复数形式的。）

We come here to learn knowledges.  

我们来这里想学知识.

Academic and policy knowledges are, in short, very much a part of the creative, knowledge economy.  

对学术和政策的认识与理解, 总的来说, 是创意知识经济很重要的一部分.

We must keep studying to obtain knowledges and temper our wills and darings.  

要不断通过学习来获取丰富的知识和培养真知灼见,以及锤炼自己的意志和胆略.

People can a relaxation in physical and mental except acquiring knowledges by tourism.  

旅游并不以简单的获取对某地的认识为目的,心理上的放松是其重要目的之一.

Deliver peer training, including both theory andteamand group members to speed growth and share knowledges.  

多次做关于编译理论和产品实现的同级培训, 以加速成长、分享知识.

To avoid the regret, I have to master knowledges and skills as much as I can.  

为了避免遗憾, 我要掌握产品知识和技能,尽我所能.

Flowing and sharing of human knowledges are a process of knowledge communication and transition actually. 

知识的流动与共享本质上是知识的交流、学习与转化的过程.

同意蒋老师的解答。knowledge为不可数名词，但表示具体的“某方面的知识”时便可数化了，可加或不加“a”,例如：

I have only (a) limited knowledge of computers. 我的计算机知识很有限。《牛津高阶英汉双解词典》

▲当knowledge表示“知道；了解”的意义时，相当于understanding，其前也可以加不定冠词“a”。如1996全国高考英语试卷第10小题：

Many people agree that ________ knowledge of English is a must in ________ international trade today . (A. a; 不填   B. the; an   C. the; the   D. 不填; the)

答案是A。句意：许多人都认为通晓英语在当今的国际贸易中是必不可少的。