Is "when" a complex word?

"When" is not a complex word. To understand why this is so, let's examine what a "complex word" is.

A complex word is a word that consists of two parts: at least one base word and either 1) an "affix" that attaches to it, like a prefix or suffix or 2) another base word that creates a compound word. 

This is a confusing concept to understand theoretically, so let's look at a few examples of complex words:

"lighting" = consists of a base ("light") and a suffix ("-ing")

"unpleasant" = consists of a base ("pleasant") and a prefix ("un-)

"blackbird" = consists of a base ("black") and another base ("bird") (This one is both a complex word and compound word)

So, with this information in mind, when we look again at the word "when," we can see it contains neither a base word nor an affix... and thus is not a complex word!