Breakdown of Ich kaufe Blumen. Wir brauchen nämlich ein Gastgeschenk.
ich
I
ein
a, an (eine)
die Blume
the flower
kaufen
to buy
Questions & Answers about Ich kaufe Blumen. Wir brauchen nämlich ein Gastgeschenk.
What does nämlich add to this sentence, and why does the English translation say "you see"?
Nämlich is an explanatory particle used to provide a reason for the statement that just happened. Here, it explains exactly why you are buying flowers. In English, we don't have a perfect single-word equivalent, but adding "you see" at the end of a sentence captures the same explanatory, conversational tone.
Can I put nämlich at the beginning of the second sentence, like "Nämlich wir brauchen..."?
No, nämlich almost never goes in the very first position. It usually sits right after the conjugated verb (in position 3), exactly as it does here: Wir brauchen nämlich... If you want to start the clause with a reason word, you have to use a conjunction instead.
Could I use denn or weil instead to explain why I'm buying flowers?
Yes, but the grammar of the sentence would have to change. Denn doesn't take up a numbered position, so you would say ...denn wir brauchen ein Gastgeschenk. Weil forces the verb to the very end: ...weil wir ein Gastgeschenk brauchen. Nämlich is handy because it lets you keep a normal, simple sentence structure while still giving a reason.
Gastgeschenk literally translates to "guest gift". Why does the translation say "host gift"?
It is all about perspective! In German, a Gastgeschenk is a gift that the guest brings to an event. In English, we usually call this a "host gift" because it is given to the host. German simply names the compound noun after the person who brings the item rather than the person receiving it.