Example: We are guarding now and we shall guard tomorrow as well:
νῦν γε φυλάττομεν καὶ φυλάξομεν αὔριον.

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 (orderly order not command.)
   πράττω or πράσσω I do πράξω
      τὸ πράγμα, τοῦ πράγματος action, deed, event, thing
      ἡ πράξις, τῆς πράξεως activity, action
      πρακτικός, ή, όν practical.
      (praxis, a pragmatic person, practical)