Learn Icelandic: Þegar strákurinn var í menntaskóla, las hann mikið. - When the boy was in high school, he read a lot.

Hvenær is a question word used to ask "When?". But here, we are making a statement about a past event. To connect two parts of a sentence meaning "at the time that...", you must use the linking word þegar.

Why is the word order las hann (read he) instead of hann las (he read) after the comma?

This is because of the V2 (verb-second) rule. The whole first clause (Þegar strákurinn var í menntaskóla) counts as the first piece of information. To keep the main verb in the second position of the main sentence, las must come right after the comma, pushing the subject hann behind it.

Why does the word menntaskóla end in an "a" here?

It is in the dative case. The dictionary form is menntaskóli. The preposition í forces the dative case when talking about a stationary location or state, like being enrolled in a school, rather than moving toward one.

Where does the word las come from?

Las is the past tense of the verb að lesa (to read) for "he/she/it". It is a strong verb, meaning it changes its inner vowel in the past tense instead of adding a regular ending like "-ði".