Slow down, you needn't drive so fast, we have plenty of time.
All good.
31 มีนาคม 2015
2
0
0
I can't express how greatful I am to you.
31 มีนาคม 2015
0
0
0
Slow down
Slow down, you needn't don't need to drive so fast, we have plenty of time.
31 มีนาคม 2015
0
0
0
This sentence looks pretty good to me. :)
You may or may not have an extra comma in this sentence, but I'll let one of the grammar professionals advise you on that ;)