Learn Icelandic: Námið var skemmtilegt þegar ég fór heim.

No, you must use þegar here. þegar is a conjunction used to connect two statements, like saying one past event happened at the time another thing happened. hvenær is only used as a question word for asking questions, like "When did you go?"

Why does the adjective "skemmtilegt" end in "-t"?

Adjectives in Icelandic must match the gender of the noun they describe. The noun here is námið (the study), which is a neuter word. The base dictionary form is skemmtilegur, but it takes the neuter ending -t to agree with the noun.

Why do we use "heim" instead of "heima" for "home"?

Icelandic uses different words for "home" depending on movement. Because the verb fór (went) shows movement towards a destination, you use the directional word heim (homewards). You would only use heima if you were already stationary at home, for example if you said "I was at home".

What is the base word for "Námið" and how does it mean "the study"?

The base word is nám, which is a neuter noun meaning "study" or "learning". In Icelandic, "the" is not a separate word before the noun but an ending attached to it. To say "the study", you add the neuter definite suffix -ið directly to the end of the word, creating námið.