I .................................. HIM ONLY ONE LETTER UP TO NOW.(SEND)FILL IN THE BLANKS WITH SUITABLE TENSES GIVEN IN BRACKETS.
Because action of the context of this sentence expresses something that was begun in the past and continues into the present, the only appropriate tense is the Present Perfect Tense. And, the Basic Form must be used because the action is not in progress (which would necessitate the Progressive Form).
- This Present Perfect Basic Form of the verb to send is formed by using the present tense of the auxiliary verb to have with the past participle of to send:
I have sent him only one letter up to now. (Incidentally, this sentence would read better if the writer begins with "up to now," by the way--Up to now, I have sent him only one letter.)
The only proper way to fill in this blank is with the phrase "have sent." You can say "I have sent him only one letter up to now." Just saying "I sent him only one letter up to now" would not be correct. You could say "I sent him only one letter," but once you add in the "up to now" you need to say "have sent."
You cannot, of course, put this in the future tense because it is talking about what has happened so far, not about what has yet to happen.