Pr.Simple: Letters are written every week.
Pr.Cont.: Letters are being written at the moment.
Pr. Perfect: Letters have already been written.
Past Simp.: Letters were written yesterday.
Past Cont.: Letters were being written at 3 o`clock yesterday.
Past Perf.: Letters had been written by 3 o`clock yesterday.
Fut. Simp: Letters will be written tomorrow.
Fut.Perf.: Letters will have been written by 3 o`clock tomorrow.
Pr.Simple: Letters are written every week.
Pr.Cont.: Letters are being written at the moment.
Pr. Perfect: Letters have already been written.
Past Simp.: Letters were written yesterday.
Past Cont.: Letters were being written at 3 o`clock yesterday.
Past Perf.: Letters had been written by 3 o`clock yesterday.
Fut. Simp: Letters will be written tomorrow.
Fut.Perf.: Letters will have been written by 3 o`clock tomorrow.