String will not form loops.
If we observe the formation of stationary wave forms of different frequencires or wave lentgths in the streched strings as shown in the given figure,we see that to produce a stationary wave the antinode is first created by plucking the string in such a way that intgral number of loops are formed in the whole length(l)of the streched string.
Now length of one loop is equal to half of wave length
#l=(nlambda)/2,"where "n =1,2,3.....#
Again the plucking of string should be done at a distance
So to creat stationary wave