Learn Dutch: Hij wil een groot boeket, maar kan er alleen twee kleine maken. - He wants a large bouquet, but can only make two small ones.

This is the quantitative er. In English, you say "two small ones", using "ones" to replace the noun. In Dutch, when you have a number (twee) but leave out the noun (boeketten), you must use er to point back to the thing you are counting.

Why does klein get an -e ending here to become kleine?

When an adjective stands alone with a number to replace a noun (like "small ones"), it almost always gets an -e ending. It acts like a noun phrase itself, so twee kleine means "two small ones".

If kleine gets an -e, why is it een groot boeket without an -e on groot?

This is a tricky rule that is easy to forget! boeket is a het-word. When a het-word comes after een, the adjective before it doesn't get an -e. So we say het grote boeket, but een groot boeket.

Why isn't the subject hij repeated after maar?