In English, you use "learnt" when it is in when it is a past participle. "I have learnt how to draw." In German, it is more complicated. Take out the "-t" and you end up with "lern" which is the stem. Ich stem+e Du stem+st/est Er stem+t Wir stem+en Ihr stem+t Sie stem+en So it goes: Ich lerne, du lernst, er lernt etc.
English is an rapidly evolving language. Don't get fooled by old books. German has a million dialects that make any non-native speaker stand out. The only rule to follow is "sounds right", if don't want to sound like you are from another planet. Writing is different, however this not a writing course
No, Gruppe (group) is a singular noun; the plural is Gruppen (groups).
Some English speakers use plural verbs with nouns that refer to a group of people, but German does not -- we don't say die Mannschaft haben gewonnen (the team have won) or die Polizei suchen den Dieb (the police are looking for the thief) but instead die Mannschaft hat gewonnen (the team has won) and die Polizei sucht den Dieb (the police is looking for the thief).
Another one that slips up with British/American English. I put "the group are learning" which is usual for British English. Teams are treated as a number of people. So "West Ham are a football team". But the American English practice is that groups are treated as singular, so "Miami Dolphins is a football team" and I got marked wrong for putting "The group are learning". I've reported it, but I just thought I'd put why I've reported it in case anyone looks.