How do you change the following sentence to the future tense?: The lake is frozen.
There are different ways on how to change the sentence to future tense.
If you want to write it in a way as if you are predicting, then:
The lake will be frozen.
Another way is:
The lake is going to be frozen.
For future perfect tense:
The lake will have been frozen.
*This can be used although it's quite rare in conversations and such.
There are different kinds of future tense, but mostly these are the ones used.