What are the difference between beliefern and liefern? I know that both mean to deliver but are they interchangeable?
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"liefern" requires info on what is being delivered, optionally to whom it gets delivered to.
"beliefern" is the other way round. You must define who gets it, but it's optional to state what gets delivered.
Note Hans Peter's examples and add the example
Blue Top Leather MCBI156002O Ishikawa Women's Sneakers Hi Wir liefern Tomaten.
for clarity of the usage.
beliefern is the transitive version of liefern. The former automatically asks for an (accusative) object. It's like the difference between antworten und beantworten:
antworten auf die Frage = die Frage beantworten