CONSONANTS

Greek words may only end in these consonants:   ν, ῥ, ς

Consonants usually assimilate

  a consonant followed by a breathing becomes aspirate:

   ἀπὸ ἧς =  ἀπ’ ἧς    by elision, and then   ἀφ’ ἧς   by assimilation.      

   a consonant in contact with one of a different order, imitates it to create a smooth transition:

  ἐν  +  μένω  >  ἐμμένω  =  to abide

Changes when endings are applied to the stem in conjugation and declension

   a dental consonant is dropped before  σ :

in the nominative singular and dative plural of the stem  ἐλπίδ-:

   ἐλπίς, ἐλπίσι(ν)

   the group formed by  ν  +  a dental drops before  σ, and the preceding consonant lengthens thus:

short  α  and short ι lengthen  into the corresponding long vowels, but

 ε lengthens   into   ει and    ο lengthens   into    ου

So, the dative plural of the stem   λεοντ-   is   λέουσι(ν)