Learn Dutch: Wij hadden een leuk spel op zolder. - We had a fun game in the attic.

Because the subject wij (we) is plural. In the simple past tense, the verb hebben (to have) becomes had for singular subjects (I, you, he, she, it) and hadden for all plural subjects (we, you all, they).

Why is it leuk spel and not leuke spel?

Spel is a het-word (het spel). A tricky but important rule in Dutch is that when a het-word follows een (a) instead of het (the), the adjective describing it does not get an -e at the end. If we were saying "the fun game", it would be het leuke spel.

Why does the sentence use op (on) instead of in for "in the attic"?

In Dutch, it is common to use op for certain spaces or floors in a house. You will frequently hear op zolder (in the attic), op mijn kamer (in my room), and op de gang (in the hallway). It is just a set preposition you have to memorize.

Why is there no word for "the" before zolder, like op de zolder?

While op de zolder is grammatically fine, it is much more natural to just say op zolder. Similar to how English speakers say they are "in bed" or "at school" without using the word "the", this is a fixed expression where the article is usually dropped.