Here you have a special sentence pattern with the verb to be. The most frequent type is:
A mouse 1 is 2 an animal 3.
Mr Smith 1 is 2 my English teacher 3.
Snow 1 is 2 white 3. Here the complement is an adjective.
The plates 1 are 2 in the cupboard 3. Here the complement is a noun group that is a where-indication.
2 verb (of the type linking verb)
3 complement (Various word classes are possible.)
Sentence patterns with the verb to be have a lot of variants.
In the subject position there can be a to-infinitive:
To err 1 is 2 human 3.
To train them 1 is 2 my cause 3.
Google shows a lot about sentence patterns or sentence structures. I don't know which are the best sites and if there is a site that has patterns only with to be and other linking verbs as to become, to stay/to remain.
One general site is: