str = StrCopy(sString,nStart,nLength)
StrCopy('Hello',2,1) = 'e'
StrCopy('Hello',2,3) = 'ell'
StrCopy('Hello',2,4) = 'ello'
StrCopy('Hello',2,1000) = 'ello'
StrCopy('Hello',1000,3) = '' // blank
You do not have to check if the values you are passing for nStart and nLength are too high. If they are, they will be ignored (see examples above).
See also : StrDelete.