well because the snakes get scared and they change color
them
It is not possible to change genders ,even if they are snakes, may change colour.
You need a subject pronoun. Anna is the subject (the doer of the action) and so is "she"-- both are performing the action in the sentence. You would only use "her" if you needed an object, the receiver of the action (The snakes bit her). Thus, it is correct to say: Anna and she (or perhaps it would sound better if you said She and Anna) studied snakes.
He
The word 'has' is not a pronoun. The word 'has' is a verb, the third person singular present of' have'. Examples: first person singular: I have some change. third person singular: He has some change. She has some change. It has no change.
2 weeks
No. Chairs would be replaced by 'it' to be an object pronoun.
If you practice, you will learn how to use pronouns. That should change the way you write.
No, "became" is not a pronoun. It is the past tense form of the verb "become" and is used to describe a change or transformation.
The pronouns for your friend are he, him, or his if your friend is a male and she, her, or hers if your friend is a female. The pronouns to use if you have more than one friend is they, them, or theirs.
Prepare is a verb. The noun form of prepare is preparation; the pronoun for preparation is it. Example use:I have a preparation to sooth sunburn; I will give it to you.