What is the difference between the impersonal 'se' and the passive 'se'?

The passive voice has got me tied up in knots. My all-spanish workbook refers to ser + particple as the formal technique. It then recommends two other forms as more common. The first is called 'se' en frase pasiva, and is se + verb in 3rd person singular or plural. However, there's also a section for a 'se' impersonal where it's se + verb in just 3rd person singular, and I can't for the life of me tell what the difference is between these 2 se + verb options?

Good question. To explain the answer, you must first understand the difference between a transitive and intransitive verb.

Though the passive ´se´ and impersonal ´se´ may look similar, they have different uses.

1.´ Impersonal se´The Spanish impersonal 'se' is the English equivalent of 'one' / 'people.' Like the English 'one,' the impersonal se refers to an unidentified human agent. It only uses intransitive verbs and 'objectless' transitive verbs as in en los Estados Unidos se come mucho 'people eat a lot in the United States (no mention of what they eat.) The verb is always singular. See below examples:

se entra por aquí	one goes in this way.
se está mejor aquí	one is better off here.

2. 'Passive se' may only be used with third-person transitive verbs, singular or plural, and usually only with non-living nouns and pronouns. It is equivalent to the passive ser. See below examples:

Es una zona de mucha sequía, así que no se ve pasto verde.	It is a very drought-ridden area, so you don´t see green grass.
se vendió la casa.	the house was sold

Identify the following sentences as impersonal or passive se.  Although capitalization does not count, spelling does!

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1. Los libros se vendían a diez euros.

2. Se enviaron los hombres y las armas necesarios para concluirla.

3. No se puede entrar.

4. Se cruza si el semáforo está en verde y se espera si está en rojo.