Tvunga is not a word, so you must be confused there.
The verb conjugates as tvinga, tvingat, tvingade i.e. weakly. Very rarely you will hear the older, strong forms with umlaut (vowel change): tvinga, tvungit, tvang. For whatever reason, tvungits (the passive form) is more common than tvungit in contemporary Swedish.
The past participle declines as tvungen, tvunget, tvungna.