He writes a letter-Present Simple
He wrote a letter yesterday-Past Simple
He will write a letter tomorrow-Future Simple
He is writing a letter at the moment-Present Continuous
He was writing a letter when I came to him-Past Continuous
He will be writing a letter at 5 o'clock tomorrow.-Future Continuous
He has already written a letter-Present Perfect
He had written a letter before he went to his work-Past Perfect
He will have written a letter by 6 o'clock tomorrow-.Future Perfect
He has already been writing a letter for half an hour-Present Perfect Continuous
