Verb + infinitive, no preposition
descendre
no prepositionde
The constructions
no prepositiondescendre + infinitif
Meaning : to go down to do
Je descends acheter le pain.
dedescendre de quelque chose
Meaning : to get off, to come down from
Elle est descendue du train.
Why
Verbs of motion (aller, venir, monter, descendre, sortir, courir) are built directly before an infinitive that states the purpose: descendre acheter le pain. Descendre de indicates what you get down from.