Read and remember.
We use the modal verbs:
V+

Read and remember.
We use the modal verbs:
V+ have + V3
to say if something was true, probable, possible or impossible in the past.
People must have lived in
Siberia in prehistoric times. Должно быть, люди жили в Сибири в доисторическое время.
You are absolutely certain that something is true. 99%
People may / might have lived in Siberia in prehistoric times. Возможно, люди жили в Сибири в доисторическое время. You think something has a good possibility of being true. 50%
People can / could have lived in Siberia in prehistoric times. Возможно, люди жили в Сибири в доисторическое время. There is a possibility that something is true.
NB: This form is not as strong as might or may. 25%
People cant / couldnt have lived in Siberia in prehistoric times. Не может быть, чтоб люди жили в Сибири в доисторическое время.You are absolutely certain that something is NOT true. 0%