Learn Dutch: Zij wilde tulpen kopen, maar ziet er alleen twee gele. - She wanted to buy tulips, but only sees two yellow ones.

Here, er acts as the quantitative "er", referring back to the noun tulpen. It translates roughly to "of them". Whenever you state a quantity (like twee) of a previously mentioned noun without repeating the noun itself, Dutch requires you to use er.

Why does the adjective have an -e ending (gele) if there is no noun after it?

The adjective gele gets an -e ending because it still modifies the implied noun tulpen. Even though the noun is dropped, the adjective keeps the exact same ending it would have if the noun were physically there (twee gele tulpen). This is how Dutch handles English phrases like "yellow ones".

Why is er placed before alleen?

Quantitative er generally moves to be as close to the conjugated verb as possible. In this clause, it places itself directly after the verb ziet, which forces the adverb alleen (only) to come after it.

Why is there no subject after maar?

The subject Zij is the same for both actions (wanting and seeing). Just like in English, you can drop the subject in the second half of the sentence to avoid repeating it. If you did include it, the normal main clause word order applies: maar zij ziet er alleen twee gele.