Breakdown of Ik vergeet tulpen te bestellen, dus ik koop er drie roze.
ik
I
kopen
to buy
drie
three
bestellen
to order
Questions & Answers about Ik vergeet tulpen te bestellen, dus ik koop er drie roze.
What is er doing in the phrase ik koop er drie roze?
This is the quantitative er, which acts a bit like "of them" in English. Whenever you state a quantity (like drie) but leave out the noun it refers to (tulpen), Dutch requires you to include er in the sentence.
How does Dutch translate "pink ones" without using a word for "ones"?
Dutch doesn't use a filler word like "ones". When you drop the noun, you just use the adjective (roze) on its own. The combination of er earlier in the clause and the lone adjective at the end tells the listener you mean "pink ones [of them]".
Why is the word order dus ik koop instead of inverting to dus koop ik?
Dus (so) can act as a coordinating conjunction, just like en (and) or maar (but). Because it acts as a bridge between two independent main clauses here, it doesn't change the standard subject-verb word order (ik koop) of the second clause.
Why do we need te before bestellen?
The verb vergeten (to forget) requires te when it is followed by the infinitive of another verb. Just like in English where we say "forget to order" instead of "forget order", Dutch links the two verbs with te.