1. Lesson 3: Grammar

2. Grammar - 替
The preposition 替 (tì) indicates that an action is performed for or on behalf of someone.
请你替我点菜

3. Grammar - 给
The preposition 给 (gěi) can mean "for," as in, "everything I do, I do it for you." You can also think of it to mean "give," like to give a service or to give an Obj.. In this case, the default position of the character is before the verb, although it sometimes comes after, depending on the verb.
The meaning of 给 is very similar to 为 (wèi) in Chinese, but 给 is more informal than 为, and there are many cases where it's simply based on convention and general practice to decide to use one or the other.

4. Grammar - 为
When you work for a company, or do other sorts of physical (and even mental) activities for another party, you're likely to use the Chinese preposition 为 (wèi). This character is often translated into English as "for," which is often unnatural or unnecessary, depending on the particular phrase.
我们 都 为 你 高兴