proc mid, dest, src, index
  push ebx

  mov ebx, [src]
  mov edx, [dest]
  add ebx, [index]
  xor ecx, ecx

@@:
  mov al, [ebx+ecx]
  mov [edx+ecx], al
  inc ecx
  test al, al
  jnz @b

  pop ebx
  ret
endp
    

Code:

proc mid, dest, src, index
  push ebx

  mov ebx, [src]
  mov edx, [dest]
  add ebx, [index]
  xor ecx, ecx

@@:
  mov al, [ebx+ecx]
  mov [edx+ecx], al
  inc ecx
  test al, al
  jnz @b

  pop ebx
  ret
endp
    

The code above is what you mean?

About moving using DWORDs the problem is that you must be sure that the string has a size multiple of 4 bytes (I mean its size, no its actual length). If the strings are actually a struc with a field holding the string length then you can move by DWORD quantities [string.len shr 2] times and then move by BYTE quantities [string.len and 3] times to move the remainder bytes.

Note that the code is actually a strcpy which starts doing what Goplat said.

invoke GlobalAlloc,GMEM_FIXED,1024
mov [temp],eax
invoke GlobalAlloc,GMEM_FIXED,1024
mov [text],eax
; ReadFile to temp
stdcall mid,text,temp,10
invoke MessageBox,0,text,text,0
    

lea ecx,[temp + 13] 
invoke MessageBox,0,temp,0,0    

;===============================================================================================
; str.sub
; desc: extract substring from string
; args: dest    - destination string buffer
;       destlen - destination string buffer length
;       src     - source string pointer
;       srclen  - source string buffer length
;       index   - index in string from where we start extracting
;       len     - number of characters to take into new string
; ret: CF set on error, otherwise
;      EAX = dest (for use with nested libcalls), and
;      OF set if index+len is behind source string (eg. dest string contains less than len chars)
; error: ERR_OUT_OF_RANGE - index is higher than length of string (or length of buffer)
;        ERR_ZERO_SIZE - destlen=0 or srclen=0 or len=0
;        ERR_BUFFER_FULL - destination buffer not long enough. in this case only first destlen chars
;                          are copied, and dest buffer is not terminated by zero
;        +str.len
;        +mem.copy
;===============================================================================================
proc str.sub, dest, destlen, src, srclen, index, len
        push    ecx esi
        pushf
        cld

        ;error if destlen=0
        cmp     [destlen], 0
        je      .error_destlen_zero

        ;error if srclen=0
        cmp     [srclen], 0
        je      .error_srclen_zero

        ;error if len=0
        cmp     [len],0
        je      .error_len_zero

        ;ECX = length of source string
        libcall str.len, [src], [srclen]
        jc      .rc
        mov     ecx, eax

        ;ERR_OUT_OF_RANGE if index >= length of source string
        cmp     [index], ecx
        jae     .error_index_out_of_range

        ;ECX = required length for the new string = min(index+len, str.len(string)) - index
        mov     eax, [index]
        add     eax, [len]
        cmp     eax, ecx
        jbe     @f
        mov     eax, ecx
        or      word [esp], 1 shl 11 ;set OF
@@:
        sub     eax, [index]
        mov     ecx, eax

        ;ECX = length in dest buffer = min(ECX, destlen)
        mov     eax, [destlen]
        cmp     ecx, eax
        cmova   ecx, eax

        ;get position in source string from where to copy
        mov     esi, [src]
        add     esi, [index]

        ;copy actual part of string that should be copied
        libcall mem.copy, [dest], esi, ecx
        jc      .rc

        ;error if there is no place for ending zero, eg. if ECX = destlen
        cmp     ecx, [destlen]
        je      .error_dest_buffer_full

        ;otherwise add ending zero
        add     ecx, [dest]
        mov     byte [ecx], 0

        ;return dest
        mov     eax, [dest]

.rnc:   and     byte [esp], not 1 ;clear CF
.r:     popf
        pop     esi ecx
        ret
.rc:    or      byte [esp], 1 ;set CF
        jmp     .r

.error_dest_buffer_full:
        mov     eax, ERR_BUFFER_FULL
        jmp     .rc

.error_destlen_zero:
        mov     eax, ERR_ZERO_SIZE
        jmp     .rc

.error_srclen_zero:
        mov     eax, ERR_ZERO_SIZE
        jmp     .rc

.error_len_zero:
        mov     eax, ERR_ZERO_SIZE
        jmp     .rc

.error_index_out_of_range:
        mov     eax, ERR_OUT_OF_RANGE
        jmp     .rc

endp