Learn Dutch: Hij heeft een vaas nodig, maar ik heb er twee grote nodig. - He needs a vase, but I need two large ones.

This is the quantitative er, which acts like "of them" in English. Because you are saying "two large ones" without repeating the word for vase (vaas), Dutch requires er to point back to the noun you are counting.

Why is it grote instead of groot?

In English, we often use the word "ones" as a placeholder (like "two large ones"). In Dutch, you simply use the adjective, but it must keep the -e ending it would normally have if the plural noun (vazen) were still sitting right next to it.

When changing groot to grote, why is one "o" dropped?

This is a classic Dutch spelling rule for keeping vowels long. The word groot has a long "o" sound. When you add the -e ending, the syllables divide as gro-te. Because that first syllable now ends in a vowel (an "open syllable"), a single "o" automatically makes a long sound. The second "o" is no longer needed.

Why does nodig appear at the very end of both clauses?

The Dutch expression for "to need" is nodig hebben (literally: to have necessary). In a standard main clause, the conjugated verb (heeft or heb) takes the second position in the sentence, while the other part of the expression (nodig) is pushed all the way to the end.