Question:
I know that I am not allowed to switch on (or off) any electrical appliances on Shabbat. But may I watch TV on Shabbat if it was left on since before Shabbat?
Answer:
On Shabbat there are 39 forms of creative activity which are forbidden.
Aside from these 39 categories of labor, it is forbidden to be involved in any activity which is considered to be a "weekday" mundane activity ("Uvdin D'Chol") Instead, we are meant to enjoy and have pleasure on Shabbat and spend our time engaged in spiritual pursuits such as praying studying and praising G-d.
For example, it is forbidden to measure on Shabbat with a measuring stick. Although this activity does not involve any of the 39 forbidden labors, it is considered to be a mundane activity.
Watching television is certainly considered to be Uvdin D'Chol, goes counter to the intended spirit of Shabbat, and is undoubtedly forbidden on the Shabbat.
