This is poor English grammar! Reported. In English, you would say either a) The physics lesson was difficult' or b) 'Physics lessons are difficult' or c) 'This physics lesson is difficult'. Nobody would ever say 'Physics lesson is difficult'.
Physics subject is not good English.
Physics is a subject.. To say physics subject is redundant sana. Haifai hata siku moja!!!!
Report it; it will get fixed quicker than commenting about it in the sentence discussion.
Apart from the translation, I think there's a major error in the Swahili text: it is either "somo la fizika ni somo gumu" or "somo la fizika ni vigumu".
No, vigumu is incorrect here. 'Difficult' in the sentence 'Physics lesson is difficult' is an (predicate) adjective. So, the use of the adjective form gumu for Ji-/Ma- class is correct!
Lessons are, lesson is