which is the longest word that can be made using the letters only on one row of the keyboard.?

Answer 1

Actually, as neat as it would be, the answer is NOT "typewriter"

The longest words are TWELVE letters long (see below).

But looking a "typewriter" for a moment. If this WERE the "longest word" at 10 letters, it would be tied with these common words:

perpetuity, proprietor, repertoire

And also with these rarer words -- all of which you'll find listed at
http://dictionary.reference.com/

pepperroot, pepperwort, pirouetter, prerequire, pretorture, tetterwort


But there are also 11 and 12-letter words.

The only one of these that is common/familiar is "teetertotter" (though some dismiss it because it is often spelled with a hyphen)
http://en.wikipedia.org/wiki/Longest_word_in_English#Words_with_certain_characteristics_of_notable_length

But here are others, both listed at dictionary.com
11 letters: rupturewort
12 letters: proterotype

The following link, and others like it written to answer your very question, lists many of the words above, as well as another 10-letter word --"repetitory"-- and two more 12-letter word:
pituitotrope, uropyoureter
http://members.aol.com/gulfhigh2/words8.html

(It also lists the 10-letter word "pewterwort", but I think that's a typo for "peterwort")

Answer 2

by using the first row... typewriter