1. I check to see if there is a noun or pronoun BEFORE "for". I found two: The pronoun "I" and the noun "letter". I choose the one CLOSEST to the word "for", which is "letter".
2. I imagine that my left hand represents that word: "letter". I imagine that my left hand IS a letter. I make my hand flat with all my fingers together, to look like a letter.
3. I then check to see if there is a noun or pronoun AFTER the word "for". There is: the pronoun "you".
4. I will let my right hand represent "you". I stick one finger up, as if it were a person.
5. Now, I hold my "letter" hand and my "you" hand in front of me and see if the word "for" tells me how to move them in relation to each other:
"letter" FOR "you".
Yes! The word "for" gives me the idea of moving the "letter" hand over toward "you".
The word "for" is a preposition.
Let's try another example:
Ex: I let the dog out.
I think the word "out" is a preposition.
1. I check to see if there is a noun or pronoun BEFORE "out". I found two: The word "I" and the word "dog". I choose the one CLOSEST to the word "for", which is "dog".
2. I imagine that my left hand represents that word: "dog". I imagine that my left hand IS a dog. I make my hand into a fist with the thumb up like a dog's head.
3. I then check to see if there is a noun or pronoun AFTER the word "out". There isn't one. Therefore, "out" doesn't pass the two-hand test. It isn't a preposition.
This is not foolproof, but it will help with many situations.
Since every preposition MUST have an object (the "right hand" object above), every preposition starts a "prepositional PHRASE". It is best to learn prepositional PHRASES rather than trying to learn a list of prepositions. This way, you will be sure you are USING each preposition correctly when you do.