On ne doit pas

Question

Why didnt we say : On ne doit pas parle la bouch pleine instead of On ne doit pas parler la bouch pleine since it is ER ending verb is that the right way ?

Accepted Answer

Bonjour Sara,
Here a link to 'devoir' in Le Présent - look at the examples and you will see that when 'devoir' is followed by a verb, it is always in the infinitive form.

Je dois parler à Sophie = I have to talk to Sophie
Il doit faire ses devoirs = he has to do his homework

The same applies with 'pouvoir' .
I hope this si helpful.
Bonne journée !

Answer

Sara, I might be a bit late but if not, DEVOIR is a helping verb like MUST, SHOULD, HAVE TO, MAY in English. He must go now not He must goes now....  In French you have to conjugate the helping verb DEVOIR and this becomes DOIT (for "one" but the infinitive (full verb) remains unchanged.  
Hence On ne doit parler ..................
Hope this helps.
 
Hence On ne doit