Learn Dutch: Gisteren zag ik een schoenendoos op zolder. - Yesterday I saw a shoebox in the attic.

This is a compound noun made of schoen (shoe) and doos (box). The rule is: if the first word has a plural form that ends only in -en (the plural of schoen is schoenen), we use the linking letters -en- between the words.

Why is the verb before the subject (zag ik) instead of ik zag?

Because the sentence starts with the time word gisteren (yesterday). Whenever a Dutch main clause begins with something other than the subject, the subject and the verb swap places. This is called inversion.

Why do we say op zolder instead of in de zolder for "in the attic"?

Prepositions often don't translate literally. In Dutch, you are physically "on" an attic (op zolder) rather than "in" it. It is also a fixed phrase that drops the article, much like how English says "in bed" instead of "in the bed".

Is zag just an irregular past tense form?

Yes, zag is the simple past tense of zien (to see). Zien is a strong, irregular verb: the singular past is zag (I/you/he/she saw) and the plural is zagen (we/you all/they saw).