Student or Learner
People are frequently astonished at the new results they achieve.
1) I understand that " achieve" above is correct and we can't use "achieved". If so, can you please help me to understand why can't we use " achieved".
2) People are frequently astonished. Is this sentence Passive Voice?
People are = present simple, third person plural
frequently = adverb
astonished = adjective
"Achieved" would refer to the past so it doesn't fit with "People are".
People were astonished at the results they achieved.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.