Future Tense: Consonants

I said insert an S, so what happens if the stem ends in a consonant instead of a vowel?
It depends on the behaviour of different consonants when followed by an s. It's all a question of sound: some work well, some don't work at all. For the Greeks, that is.
Let's concentrate on those that work well first:

1. Dental             δ  or  ζ  or  θ  or  ττ (also often σσ) (some)  ?
   These letters are usually dropped and replaced by the  σ  
   Called dental because the tongue touches our teeth when we pronounce them. (from Latin dens, dentis meaning tooth of course)
   

   today, normally, right now		in future
   σπεύδω		σπεύσω
   πείθεις (persuade)		πείσεις
   ἀγοραζει (shop, buy)		ἀγοράσει
   ἀναγκάζομεν (force)		ἀναγκάσομεν
   πείθεσθε (obey)		πείσεσθε
   παρασκευάζονται  (get [themselves] ready)		παρασκευάσοvtai

    Note  For some apparent exceptions, see here
   And for verbs with stem ending in -ίζω/-ίζομαι see here.

2. Labial            β or π / πτ or φ  ?
   In writing these letters are never followed by σ so the sounds βσ, ps and fs are all written:  ψ  
   Called labial because our lips do all the work this time. (from Latin labium meaning lip)
   

   today, normally, right now		tomorrow, in the future
   βλέπω		βλέψω
   γράφεις		γράψεις
   κατατρίβει (wears out)		κατατρίψει
   προσάπτομεν (attach)		προσάψομεν
   σκάπτετε (dig)		σκάψετε
   λείπουσιν		λείψουσιν
   ἕπομαι		έψομαι
   ῥίπτεται  (throws himself)		ῥίψεται
   τρέπονται (turn)		τρέψονται

3. Guttural            γ  or  κ  or  χ  or  ττ (most)  ?
   Again, in writing these letters can't be followed by s so the sounds gs, ks and khs are all written:  ξ  
   Called 'guttural' (from Latin guttur meaning throat) because they are pronounced in our throat ( χ ), or at least very near it (back of tongue against palate, γ and κ)
   

   today, normally, right now		tomorrow, in the future
   λέγω		λέξω
   διώκεις		διώξεις
   παρέχει		παρέξει
   φυλάττομεν		φυλάξομεν
   ἄγετε		άξετε
   ἀνοίγουσιν		ἀνοίξουσιν
   δέχομαι		δέξομαι
   εὔχῃ		εὔξῃ
   ἕλκεται (drags itself)		ἕλξεται

   Oops, what is that  φυλάξομεν doing here? see here.
   As strange as it may seem, the original stem of
      φυλάττω is φυλακ , as shown quite clearly by
   ὁ φύλαξ,, τοῦ φύλακος the guard (lesson six)

   Most verbs in ττ often spelt σσ even, originating from γ or κ stemmed forms follow the same pattern:
      τάττω or τάσσω I place (in order) τάξω 
         ἡ τάξις, τῆς τάξεως the rank, order (in order) not command.
      πράττω or πράσσω I do πράξω
         τὸ πράγμα, τοῦ πράγματος action, deed, event, thing
         ἡ πράξις, τῆς πράξεως same meaning
         πρακτικός, ή, όν practical.
         (praxis, a pragmatic person, practical)