How to write impersonal constructions with SE in the present tense?

In the question: en las libraries SE VENDEN libros y revistas is the statement.
I understand the use of SE and i understand it is a general statement hence the impersonal. However how do i know the root ending of the word? -en?
Another example: En costa Rica SE HABLA espaol. How do I know it ends in habla and not hablan?

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