Some sentences! ;)

1) I think that he hasn't received [1] our letter yet. We had sent it just the day before. [2] Anna offers/offered to phone him tonight.
2) The teacher wondered whether we had finished the lab work or not.
3) She complained to her sister that she would be able the next day. [??  3] But she promised that she would go to see/visit them the next month.
4) They reminded her that Mr. Braun must talk with representatives of the firm and convince them to begin to work with us.
5) That day the doctor has said [4] that I should be in hospital for several days. He pointed out/recommended/ordered that I should take meds 3 times a day.

[1]  I think you meant "received" rather than "taken."  However, without context, I can't be sure.

[2]  If you mean you sent it вчера:  "We sent it just yesterday."

[3]  Sorry, I can't understand the meaning of the sentence.  If you paraphrase in English, or write the sentence in Russian, I'll try again to help.

[4]  Don't use the present perfect tense ("has said") when the sentence mentions a definite time something happened in the past ("that day").   Exception:  You do use the present perfect tense when an action started in the past and continues until the present.  "I have lived here since March 1, 1995."