what is the difference between 替 and 为？

“替”一般表示代替某人做某事，比如“我生病了，你替我去上课吧”、“我今天有事不能去，他替我去”。“为”一般表示目的，“爸爸为了这个家做了很多”、“为了去中国留学，我努力学习中文”，

1. "替": when we express " help someone to do what they were supposed to do" use "替"
eg. 他替我参加会议/he attended the meeting in my place.(replace me)
2. "【为】": when we express" acting for someone's benefit, purpose, or 【emotion】" use "为"
eg. 医生为病人检查身体/ The doctor examined the patient for their sake（serving the patient）
他为我准备了礼物/he prepared a gift for me（to make me happy）
我为你【感到骄傲】/I feel proud of you【（emotional connection）】

为 for (in the sense of in order to or with the intention of)
替 for (with the idea of substitute, replace, on behalf of)
So I have the following questions:
我听说一个同事说“她常常为同事们帮忙”，那我还以为这里不得不使用“给”=〉她常常给同事们帮忙；如果我的句子也对了，那这两句有什么不一样的呢？？
替的差别看起来明显，就是substitute/replace的意义，例如“替人值班”。 可是用起来呢，一点简单也没有。。。好像以下的例子都能说：
这是我给朋友买的礼物
这是我为朋友买的礼物
这是我替朋友买的礼物
只有每一句的第4个字不一样，麻烦大家帮我翻译一下(I really wish to understand more clearly)

When 替 is used to express emotions, it's really similar with 为, and we usually can use either in daily life.

However, 替 shows the implied meaning of "I am thinking from your perspective", which comes from the 代替 meaning. For example, 我替你感到高兴, it means "I'm happy for you (If I were you, I'd also feel happy)."

Here if we use 为, 我为你感到高兴, it has the same meaning "I'm happy for you" but no the implied meaning. Because 为 simpliy means "for you".

In real life, this difference can be ignored, and you can use either of these two at will.

BTW, when we use this sentence, we usually say in two ways:
1. 我替你感到（很）+ feeling。
2. 我替你 + feeling。
So in your example, we'd better to delete 很, or add 感到.