Actually, while 이것 is correct,
이거 is more commonly used in real life (according to Talk To Me In Korean). It should be noted that duolingo only accepts 이것.
what the audio said sounded a lot like the ball is itchy, but the sentence said it was dirty. my mom said it was 'deo', not 'ga'. Is this the new Yanny/Laurel?
it depends on whether the verb ends with -하다 or -다. if it ends with a consonant, the ending would be 습니다, but if it ends with a vowel the ending would be ㅂ니다, so 먹다 would be 먹습니다, 보다 would be 봅니다, and 공부하다 would be 공부합니다.