Learn German: Ich lese nur dasjenige, was du mir vorschlägst. - I only read the one n that you suggest to me.

While you could simply say Ich lese nur das, was..., using dasjenige puts special emphasis on the specific thing being pointed out. It acts as a strong pointer directly linking to the relative clause that follows.

Why does the relative clause start with was instead of das?

When referring to a general, unspecified neuter pronoun like dasjenige, das, or alles (everything), German uses was as the relative pronoun. If we were referring to a specific neuter noun, like dasjenige Buch (that particular book), we would use das instead.

What case is dasjenige in here, and how does its ending work?

It is in the accusative case because it is the direct object of lese (read). The word is made of two parts that both decline: the definite article das and the adjective jenige. Because neuter nominative and accusative are the same, the form remains dasjenige.

Why is the separable prefix in vorschlägst attached to the verb instead of at the end?

The verb vorschlagen (to suggest) is a separable verb. Normally the prefix vor- would jump to the end of the clause, but because this is a subordinate relative clause, the conjugated verb is pushed to the very end, causing the prefix to stay firmly attached.