The Completed sentences are:
- Do you believe in changing?
- To which family of animals do they belong?
- We think our planet is changing.
- I hope the lake is warm!
- The mountain air smells so good!
- I hate seeing animals in zoos. They don't have much space.
In terms of the first sentence: Do you believe in changing?, In this sentence, the present simple tense is used in the question form. The base form of the verb "believe" is used because it is in the present simple.
Lastly, In terms of the second sentence: To which family of animals do they belong?, Here, the present simple is used in the question form. The base form of the verb "belong" is used. The pronoun "they" refers to a plural subject, and "belong" is used to match the present simple tense.