I believe ONLY Season 3 and some of Season 4 was filmed in front of a Live Studio Audience. Why they stop that? It seemed to be very successful? I thought it was a great idea and it made they show really funny. It might have been one of the best seasons?

Uhhh I'm confused. I think the only two seasons that WASN'T filmed in front of a live studio audience is seasons 1 and 2.

Listen in the start of the show.
Example: s3ep40 Happy Days Fonzie Moves In
Live audience (all of Season 3)

Sometimes they also said filmed in front of a Live Studio audience, so I am guessing that is the same as Live audience?

Example: s4ep68 Happy Days Fonzies Hero
Happy Days was filmed in front of a Studio audience. Does that mean the laughtrack was later added?

Does Live audience and studio audience mean the same thing?

I am guess where nothing was said, the laughter was added.
Example: s5ep88 Happy Days Hollywood Part 1

season 3-on was filmed in front of a live studio audience. (with the exception of 1 ep in season 2 and the location shots for special episodes)

they just weren't "live" enough for the producers, so they added a laugh track on top as well.

*More Confused Now*

There was one experimental episode ("Fonzie's Getting Married") in Season 2 that was filmed in front of an audience using the traditional 3 camera set-up but outside of that, from season 3 on, the rest of the series was filmed in front of a live studio audience for live laughter responses. As stated by the posters above, when they went on locations (such as the California episodes or the dude ranch or anything "outside" of the studio) they couldn't take the 300 people from the audience with them, so they simply shot it like a film (with one camera), edited it together and added a laugh track (to tell you when to laugh).

Seasons 1 & 2 were shot film style, with one camera and a "laugh track" added.

I recently saw some season 11 episodes and those were most definitely not filmed in front of an audience at all, though they unfortunately saw fit to dub in the screaming of teenage girls for Henry Winkler and Scott Baio's entrances. The season 10 episode featuring Ken Osmond also pretty obviously was 100% sweetened, i.e. if there really was an audience for the filming of that one they sat in stony silence throughout!

Happy Days is very far from unique in this regard. A number of shows that that traditionally filmed in front of an audience ceased to do so at some point for whatever reason. The Jeffersons and Webster immediately come to mind as examples, but there were others.

I believe there were some episodes that were definitely not shot in front of a studio audience, such as the Hollywood eps like antman67 mentioned, and the Westward Ho! eps. Those episodes were mostly shot on location and the studio scenes that needed to be filmed were minimal. Also the clip shows and the musical episodes (Be My Valentine, American Musical) appear to have been filmed without a studio audience.

Maybe this explains why I heard Aaron Williams or whoever laughing in the audience in episodes.

Listen in the start of the show.
Example: s3ep40 Happy Days Fonzie Moves In
Live audience (all of Season 3)

Sometimes they also said filmed in front of a Live Studio audience, so I am guessing that is the same as Live audience?

Example: s4ep68 Happy Days Fonzies Hero
Happy Days was filmed in front of a Studio audience. Does that mean the laughtrack was later added?

Does Live audience and studio audience mean the same thing?

I am guess where nothing was said, the laughter was added.
Example: s5ep88 Happy Days Hollywood Part 1

I've always assumed that there's no difference between live audience and studio audience and that they're called either one.