Просклонять по всем временам: she writes a letter.

She is writing a letter Pr Cont.
She will write a letter Future Simple
She wrote a letter Past Simple
это необязательно, думаю, времен в английском много, но на всякий случай ещё 3:
She has written a letter Pr Perfect
She had written a letter Past Perfect
She will be writing a letter Future Cont