Dash with "en"
When "en" is placed at the beginning of a sentence, there is no hyphen with the verb:
- en venant
- en marchant
- s'en aller
On the other hand, when the pronoun is placed right after the verb, a hyphen is put between the verb and the pronoun:
- viens-en au fait
- vends-en un peu
When the verb does not end with an s, an s is added at the end of the verb to take into account the sounds. This is said to be an euphonic reason:
- donnes-en mais donne
- gardes-en un peu mais garde-les
At the imperative with the personal pronoun of the second person (you), the pronoun changes to t' in front of the en :
- va-t'en
- convaincs-t'en
Dash with "y"
With the pronoun y, we find the same rules as with en.
- vas-y mais va
- rends-t'y
- allons-y
- nous y allons
Dash with "non"
When no is followed by a verb at the infinitive, a hyphen is added to the verb. If the verb is not at the infinitive, there is no hyphen.
- une fin de non-recevoir
- un devoir non achevé
Note that this is only valid for a verb, since for nouns, you can have a hyphen:: un non-dit (an unspoken), une zone de non-droit (a no-law zone), un non-fumeur (a non-smoker), etc.