To… / for… / so that…

1. "To" as an infinitive marker:

   The word "to" is commonly used as an infinitive marker in English. It is used before the base form of a verb (infinitive) to indicate purpose, intention, or the result of an action. 

Here are some key uses of "to":

   - Expressing purpose or intention: "I went to the store to buy groceries."

   - Expressing the result of an action: "She studied hard to pass the exam."

   - After certain adjectives: "It's difficult to understand the instructions."

   - After certain verbs: "He wants to learn how to play the piano."

2. "For" as a preposition:

   The preposition "for" is used to indicate the purpose, aim, or intended recipient of an action. Here are some common uses of "for":

   - Indicating purpose: "I bought a gift for my sister's birthday."

   - Indicating a recipient or beneficiary: "I made dinner for my family."

   - Expressing duration: "He worked late for several hours."

   - Expressing a cause or reason: "She was praised for her hard work."

3. "So that" as a conjunction:

   "So that" is a conjunction used to indicate the purpose or the desired outcome of an action. It is usually used to explain the reason behind someone's action. Here are some examples of using "so that":

   - Expressing purpose: "She studied hard so that she could get good grades."

   - Indicating the intended result: "He left early so that he wouldn't miss the train."

   - Giving an explanation or reason: "I wrote a note so that you would remember the instructions."

In summary, "to" is used to express purpose or intention, "for" indicates the purpose or intended recipient of an action, and "so that" explains the desired outcome or reason behind an action. Understanding these distinctions will help you use these words correctly in various contexts.