1. letters sent (everyday, now, every week)
2. letters sent (yesterday, today, before you come)
3. letters sent (when i came, yesterday at this time)
4. letters will be sent ( tomorrow, before you come)
It is difficult to be a parent.
It is dangerous to swim along if you can't swim.
It is impossible to breath under the water.
It is stupid to sing in midnight.
It is boring sitting along all day.