The pronoun that could replace "snakes" is "they." This is because "snakes" is a plural noun, and "they" is the corresponding plural pronoun used to refer to a group of entities. For example, instead of saying "Snakes are slithering," you could say "They are slithering."
them
well because the snakes get scared and they change color
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.
Using an object pronoun, the sentence would be:'The game wardens noticed it.'