When talking your native language you might find yourself at a loss for the right word sometimes and may use a filler word, such as um or err in English, to give you a bit more time to think of the word. Or you might use other words in various situations, such as ouch! or ow! when you hurt yourself, oops! when you drop something or knock something over, or yuck! when you taste something unpleasant. It’s very useful to learn such fillers and interjections in foreign languages you’re learning. One way to do so is to listen to a lot of ordinary conversation and to pay attention to these fillers.
If French you could say Aïe! or Ouille! when you hurt yourself, Miam miam! for Yum yum! (that tastes good), Beurk! or Burk! for Yuck!, and Alors! or Eh bien! for Well!. If words temporarily escape you, you can say Ben!, Euh! or Heu!, and if you’re not sure what someone else is talking about, you can express this by saying Hein?
You can indicate that you think someone is talking nonsense by saying Bah! or Tratata!, but if this doesn’t really worry you, you could say Tant pis! (if can’t be helped). If you want to express your frustration in a mild kind of way, you could say Sacrebleu!, Sacredié!, Sacredieu! or my favorite French expression, Zut! / Zut alors!