A "Fernseher" is a television set, i.e. the electronic device you need to watch TV. "Er war oft im Fernseher" would mean something like "He was often in/inside the television set". To talk about something shown on TV, you have to say "im Fernsehen" in German.
Thanks! I had the same question about the difference between "Fernseher" and "Fernsehen." Your response was so helpful! Duo should include this distinction in the translation--using "television set" for Fernseher.
Surely im is 'in dem', ie 'on the TV' not 'on tv' which was the correct answer?
Yes, "im" is short for "in dem" (in the). However, in German you sometimes use the contracted forms where you wouldn't use a definite article in English. It's impossible in German to say "in Fernsehen"; if you want to say that a person appeared on TV you have to say "im Fernsehen".
The same is true for months, days of the week and seasons: it's always "im Januar" (= in January), "am Montag" (= on Monday) and "im Sommer" (= in summer); you can't say "in Januar", "an Montag" or "in Sommer".
Languages handle the use of articles differently. While German needs it in this instance (as part of the contracted preposition), English can do without.
But is it really 'English can do without', or English does not actually work with the article at all?
In English, you would say "He was often on television" - no article for television, used in this sense.
"Often" and "regularly" are not quite the same thing. "Often" is about frequency. "Regularly" suggests some order and routine timing.
"He was on TV regularly" suggests he was on TV on approximately evenly spaced intervals - for example, the first Monday of every month, or every three days, or once a month, or approximately twice a week (even if not on the same days). He may be on TV regularly, without necessarily being on TV often.
And he can be on TV often, without necessarily being on TV regularly. For example, a nationally known expert may be on television often, whenever an issue or event arises in his/her area of expertise. But that might not happen "regularly."
[US Native English speaker]