Different tenses in personal passive constructions

1.    Main clause present + that-clause present/future => present infinitive 

a) It seems that they are very busy. 
     → They seem to be very busy.

b) It seems that they will know it.
     → They seem to know it.

2.  Main clause present + that-clause past/present perfect => perfect infinitive 

a) It seems that he was ill.
    → He seems to have been ill.

 b) People think he has left the family.
    → He is thought to have left the family.

3.  Main clause past tense + that-clause past tense => present infinitive 

It seemed that he was ill.
→ He seemed to be ill.

4. Main clause past tense + that-clause past perfect => perfect infinitive 

It seemed that he had been ill.
→ He seemed to have been ill.