Надо просклонять ПИСАТЬ ПИСЬМО
По временам ниже

to write a letter

1) I, you, we, they write a letter; he, she writes a letter

2) I, you, he, she, we, they wrote a letter

3)  I, you, he, she, we, they will write a letter

4) I`m,  you are, he is, she is, we are,  they  are writing a letter

5) I was, you were, he was, she was, we were, they were writing a letter

6)  I will (shall) be, you will be, he will be, she will be, we will (shell) be, they will be  writing a letter

7) I have, you have, he has, she has, we have, they have written a letter