To recite with tajweed is wajib in salaah.
However whilst reciting what if a mistake is made [i:8d5d522dea]knowingly[/i:8d5d522dea], but one cannot immediatly correct it. So for example one is reading and lets say he is about to recite a letter ha (the heavy ha from the chest) but becasue he cannot understand arabic he forgets whether it was the heavy ha from the chest or the one from the throat he should be reciting. What does he do, which ha should he recite? What if he recites the wrong ha, he is in salaah so he cannot check it immediatly. what if afterwards he checks it and realises that he recited the wrong ha, is his salaah invalid and he must repeat? This also goes for many other letters such as Saad and saa or Tha and taa etc.
So must every letter be correctly pronouced from the correct place of origin or if it sounds something like the correct letter it is fine to read it like that. But if this is the case then there would be no differnce in similar sounding letters as for example Saad and saa would be read without distinguishment in salaah.
May Allah reward you.